CentOSでRedmineのE-R図出力のため、railroadyのインストールをしようとするとエラーが出力されてしまいます。

Question

railroadyでRedmineのE-R図を出力しようとしましたが、
下記エラーが出現して出力できません。
（事前にGraphviz、railroadyを）
Googleで検索すると似たようなエラーとその対処法が報告されているページが
散見されましたが、如何せんRubyやLinux初心者（触って1週間程度）ですので、
具体的な対処法が思い浮かばず途方に暮れてしまいました。

どなたかアドバイスを頂ければと存じます。

[root@localhost htdocs]# yum install git

[root@localhost htdocs]# yum install graphviz

[root@localhost htdocs]# yum install rubygems

[root@localhost htdocs]# gem install railroady

[root@localhost htdocs]# railroady -o controller.dot -C
Error loading application environment.
  (Are you running railroady on the application's root directory?)
Traceback (most recent call last):
    15: from /root/.rbenv/versions/2.5.0/bin/railroady:23:in `<main>'
    14: from /root/.rbenv/versions/2.5.0/bin/railroady:23:in `load'
    13: from /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/railroady-1.5.3/bin/railroady:53:in `<top (required)>'
    12: from /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/railroady-1.5.3/lib/railroady/app_diagram.rb:42:in `process'
    11: from /root/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/railroady-1.5.3/lib/railroady/app_diagram.rb:68:in `load_environment'
    10: from /root/.rbenv/versions/2.5.0/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'
     9: from /root/.rbenv/versions/2.5.0/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'
     8: from /opt/redmine-3.4.6-0/apps/redmine/htdocs/config/environment.rb:2:in `<top (required)>'
     7: from /root/.rbenv/versions/2.5.0/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'
     6: from /root/.rbenv/versions/2.5.0/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'
     5: from /opt/redmine-3.4.6-0/apps/redmine/htdocs/config/application.rb:1:in `<top (required)>'
     4: from /root/.rbenv/versions/2.5.0/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'
     3: from /root/.rbenv/versions/2.5.0/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'
     2: from /opt/redmine-3.4.6-0/apps/redmine/htdocs/config/boot.rb:4:in `<top (required)>'
     1: from /root/.rbenv/versions/2.5.0/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'
/root/.rbenv/versions/2.5.0/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require': cannot load such file -- bundler/setup (LoadError)
[root@localhost htdocs]# 
​

【環境】
OS：CentOS 7
Redmine：3.4.6-0（Bitnamiでインストール）