GradleプロジェクトをHerokuへデプロイし、アクセスしたところ「java.lang.ClassNotFoundException: org.postgresql.Driver」がでてしまう

GradleプロジェクトをHerokuへデプロイし、アクセスしたところ「java.lang.ClassNotFoundException: org.postgresql.Driver」がでてしまいます。

java.lang.ClassNotFoundException: org.postgresql.Driver
とのことなので、クラスパスの設定がうまくいっていないと思っているのですが、Gradle初心者のためどのようにしてクラスパスを通せるのかわかりません。
Eclipse上では「ビルドパス」「外部アーカイブの追加」を実施して「postgresql-42.1.4.jre7.jar」を追加しています。

初歩的な質問かもしれませんが、アドバイスなど頂けますと幸いです。
よろしくお願いいたします。

//プログラム（一部抜粋）
Connection db = null;
PreparedStatement ps;
ResultSet rs;
// PostgreSQL JDBC ドライバロード
try {
Class.forName("org.postgresql.Driver");
// PostgreSQL JDBC 接続
String dsn =
"postgres:XXXXX";
db = DriverManager.getConnection(dsn);
}catch (Exception e) {
// エラー処理
System.err.println("SQL failed.");
e.printStackTrace ();
}

build.grade
dependencies {
// This dependency is exported to consumers, that is to say found on their compile classpath.
api 'org.apache.commons:commons-math3:3.6.1'

// This dependency is used internally, and not exposed to consumers on their own compile classpath.
implementation 'com.google.guava:guava:23.0'

// Use JUnit test framework
testImplementation 'junit:junit:4.12'

compile group: 'commons-collections', name: 'commons-collections', version: '3.2'
compile group: 'javax.servlet', name: 'javax.servlet-api', version: '3.1.0'
testCompile group: 'junit', name: 'junit', version: '4.+'
compile group: 'org.postgresql', name: 'postgresql', version: '9.0-801.jdbc4'
 runtime('org.postgresql:postgresql')

}

ログ
2017-11-09T14:19:14.376642+00:00 heroku[router]: at=info method=GET path="/hello" host=pacific-bastion-55589.herokuapp.com request_id=443651b0-f86d-4b11-a29f-6811e57f97ec fwd="60.120.49.207" dyno=web.1 connect=0ms service=14ms status=500 bytes=1518 protocol=https
2017-11-09T14:19:14.368425+00:00 app[web.1]: SQL failed.
2017-11-09T14:19:14.368589+00:00 app[web.1]: java.lang.ClassNotFoundException: org.postgresql.Driver
2017-11-09T14:19:14.368811+00:00 app[web.1]: at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1291)
2017-11-09T14:19:14.368918+00:00 app[web.1]: at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1119)
2017-11-09T14:19:14.368998+00:00 app[web.1]: at java.lang.Class.forName0(Native Method)
2017-11-09T14:19:14.369116+00:00 app[web.1]: at java.lang.Class.forName(Class.java:264)
2017-11-09T14:19:14.369489+00:00 app[web.1]: at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
2017-11-09T14:19:14.369223+00:00 app[web.1]: at pack.Contoller.doGet(Contoller.java:44)
2017-11-09T14:19:14.369302+00:00 app[web.1]: at javax.servlet.http.HttpServlet.service(HttpServlet.java:635)
2017-11-09T14:19:14.369410+00:00 app[web.1]: at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
2017-11-09T14:19:14.369595+00:00 app[web.1]: at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
2017-11-09T14:19:14.369679+00:00 app[web.1]: at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
2017-11-09T14:19:14.369788+00:00 app[web.1]: at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
2017-11-09T14:19:14.369906+00:00 app[web.1]: at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:478)
2017-11-09T14:19:14.369993+00:00 app[web.1]: at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
2017-11-09T14:19:14.370098+00:00 app[web.1]: at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
2017-11-09T14:19:14.370209+00:00 app[web.1]: at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
2017-11-09T14:19:14.370294+00:00 app[web.1]: at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
2017-11-09T14:19:14.370403+00:00 app[web.1]: at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803)
2017-11-09T14:19:14.370540+00:00 app[web.1]: at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
2017-11-09T14:19:14.370618+00:00 app[web.1]: at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868)
2017-11-09T14:19:14.370728+00:00 app[web.1]: at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1459)
2017-11-09T14:19:14.370805+00:00 app[web.1]: at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
2017-11-09T14:19:14.370909+00:00 app[web.1]: at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
2017-11-09T14:19:14.371007+00:00 app[web.1]: at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
2017-11-09T14:19:14.371117+00:00 app[web.1]: at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
2017-11-09T14:19:14.371234+00:00 app[web.1]: at java.lang.Thread.run(Thread.java:748)
2017-11-09T14:19:14.373085+00:00 app[web.1]: at pack.Contoller.doGet(Contoller.java:58)
2017-11-09T14:19:14.373082+00:00 app[web.1]: Nov 09, 2017 2:19:14 PM org.apache.catalina.core.StandardWrapperValve invoke
2017-11-09T14:19:14.373086+00:00 app[web.1]: at javax.servlet.http.HttpServlet.service(HttpServlet.java:635)
2017-11-09T14:19:14.373083+00:00 app[web.1]: SEVERE: Servlet.service() for servlet [pack.Contoller] in context with path [] threw exception
2017-11-09T14:19:14.373084+00:00 app[web.1]: java.lang.NullPointerException
2017-11-09T14:19:14.373087+00:00 app[web.1]: at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
2017-11-09T14:19:14.373088+00:00 app[web.1]: at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
2017-11-09T14:19:14.373089+00:00 app[web.1]: at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
2017-11-09T14:19:14.373088+00:00 app[web.1]: at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
2017-11-09T14:19:14.373090+00:00 app[web.1]: at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
2017-11-09T14:19:14.373090+00:00 app[web.1]: at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:478)
2017-11-09T14:19:14.373091+00:00 app[web.1]: at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
2017-11-09T14:19:14.373092+00:00 app[web.1]: at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
2017-11-09T14:19:14.373092+00:00 app[web.1]: at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
2017-11-09T14:19:14.373094+00:00 app[web.1]: at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
2017-11-09T14:19:14.373093+00:00 app[web.1]: at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
2017-11-09T14:19:14.373095+00:00 app[web.1]: at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868)
2017-11-09T14:19:14.373094+00:00 app[web.1]: at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803)
2017-11-09T14:19:14.373097+00:00 app[web.1]: at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
2017-11-09T14:19:14.373096+00:00 app[web.1]: at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1459)
2017-11-09T14:19:14.373096+00:00 app[web.1]: at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
2017-11-09T14:19:14.373099+00:00 app[web.1]: at java.lang.Thread.run(Thread.java:748)
2017-11-09T14:19:14.373098+00:00 app[web.1]: at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
2017-11-09T14:19:14.373098+00:00 app[web.1]: at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
2017-11-09T14:19:14.373100+00:00 app[web.1]: