[#42296389] Added option to specify bundler version in the rbenv::version

This change would allow you to specify the version of bundler to be
installed with the ruby version. Not specifying the version leads to
installing the version available by default for bundler from rubygems.
The format:

  rbenv::version { '1.9.3-p392':
    bundler_version => '1.1.4'
  }
[#42296389] Made tests more strict and added context around a test

Also fixed a warning
Added documentation around rbenv::version usage
making format for doc better :bowtie:
Merge pull request #1 from alphagov/bundler_version

[#42296389] Added option to specify bundler version in the rbenv::version