SpringBootアプリのHerokuデプロイ時にspring-boot-gradle-pluginが取得できない

Question

バージョン情報

• JDK 1.8.0_172
• Gradle 4.10

解決したいこと

HerokuにSpringBootで作ったアプリをデプロイしようとしたのですが、
下記のエラーが出てソースコードのpushに失敗します。

-----> Gradle Wrapper app detected
-----> Installing OpenJDK 1.8... done
-----> Building Gradle app...
       WARNING: The Gradle buildpack is currently in Beta.
-----> executing ./gradlew
       Downloading https://services.gradle.org/distributions/gradle-4.10-all.zip
       ...............................................................................................................
       
       Welcome to Gradle 4.10!
       
       Here are the highlights of this release:
        - Incremental Java compilation by default
        - Periodic Gradle caches cleanup
        - Gradle Kotlin DSL 1.0-RC3
        - Nested included builds
        - SNAPSHOT plugin versions in the `plugins {}` block
       
       For more details see https://docs.gradle.org/4.10/release-notes.html
       
       Starting a Gradle Daemon (subsequent builds will be faster)
       
       FAILURE: Build failed with an exception.
       
       * What went wrong:
       A problem occurred configuring root project 'KakeiboApp'.
       > Could not resolve all artifacts for configuration ':classpath'.
          > Could not resolve org.springframework.boot:spring-boot-gradle-plugin:2.0.5.RELEASE.
            Required by:
                project :
             > Could not resolve org.springframework.boot:spring-boot-gradle-plugin:2.0.5.RELEASE.
                > Could not get resource 'https://repo.maven.apache.org/maven2/org/springframework/boot/spring-boot-gradle-plugin/2.0.5.RELEASE/spring-boot-gradle-plugin-2.0.5.RELEASE.pom'.
                   > Could not GET 'https://repo.maven.apache.org/maven2/org/springframework/boot/spring-boot-gradle-plugin/2.0.5.RELEASE/spring-boot-gradle-plugin-2.0.5.RELEASE.pom'.
                      > Received fatal alert: protocol_version
       
       * Try:
       Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.
       
       * Get more help at https://help.gradle.org
       
       BUILD FAILED in 12s
 !     Failed to build app
 !     Push rejected, failed to compile Gradle Wrapper app.
 !     Push failed

build.gradleに記載したspring-boot-gradle-pluginが取得できていないようなのですが、
どのように修正すればよいのか分からずに詰まっています。

もし解決策をご存知の方がいらっしゃいましたら、ご教示頂けますと幸いです。

各種設定ファイル

build.gradle

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:2.0.5.RELEASE")
    }
}

apply plugin: 'java'
apply plugin: 'eclipse'
apply plugin: 'idea'
apply plugin: 'org.springframework.boot'
apply plugin: 'io.spring.dependency-management'

bootJar {
    baseName = 'gs-serving-web-content'
    version =  '0.1.0'
}

repositories {
    mavenCentral()
}

sourceCompatibility = 1.8
targetCompatibility = 1.8

dependencies {
    compile("org.springframework.boot:spring-boot-starter-web")
    compile("org.springframework.boot:spring-boot-starter-thymeleaf")
    compile("org.springframework.boot:spring-boot-devtools")
    compile("org.springframework.boot:spring-boot-starter-jetty")
    // JPA Data (We are going to use Repositories, Entities, Hibernate, etc...)
    compile 'org.springframework.boot:spring-boot-starter-data-jpa'
    // Use MySQL Connector-J
    compile 'mysql:mysql-connector-java'
    compile 'postgresql'
    testCompile("junit:junit")

}
//デプロイ時のデフォルトタスク
defaultTasks "clean", "build"
task stage(dependsOn: ['build', 'clean'])
build.mustRunAfter clean

system.properties

java.runtime.version=1.8.0_172

Procfile

web: java -jar build/libs/KakeiboApp-1.0.jar --server.port=$PORT

ディレクトリ構成

.
├── .gitignore
├── .system.properties
├── Procfile
├── build.gradle
├── setting.gradle
├── gradlew
├── build
│   ├── classes
│   │   └── java
│   │       └── main
│   │           └── income_outgo
│   ├── libs
│   ├── resources
│   │   └── main
│   │       ├── static
│   │       │   ├── css
│   │       │   ├── img
│   │       │   └── js
│   │       └── templates
│   │           ├── category
│   │           └── income_outgo
│   └── tmp
│       ├── bootJar
│       └── compileJava
├── gradle
│   └── wrapper
├── out
│   └── production
└── src
    ├── main
    │   ├── java
    │   │   └── income_outgo
    │   └── resources
    │       ├── static
    │       │   ├── css
    │       │   ├── img
    │       │   └── js
    │       └── templates
    │           ├── category
    │           └── income_outgo
    └── test
        ├── java
        └── resources