Put Java into the path for jruby gems

Issue
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Trying to [install "bundler" for all Rubies](https://github.com/boxen/puppet-ruby/commit/1794fa17b04cd19a68365ebc272e6b2a71c7185c) currently results in:

> ```
Puppet (err): Execution of '/opt/rubies/jruby-1.7.4/bin/gem install 'bundler' --version '~> 1.0' --source 'https://rubygems.org/' --no-rdoc --no-ri --verbose' returned 255: No `java' executable found on PATH.
```

...if `jruby` is one of your rubies!

But `java` is installed in `/opt/boxen/bin` by https://github.com/boxen/puppet-java for jruby (see https://github.com/boxen/puppet-ruby/blob/master/manifests/version.pp#L28 ).

Change
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Add Boxen's `bin` directory to `${PATH}` when installing gems if the Ruby version is a `jruby` version.

[The start of the conversation](https://github.com/boxen/puppet-ruby/blob/master/CONTRIBUTING.md)
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This could really do with a spec, but I have no idea where to start. I'll hang out in #boxen for a while should anyone (@dgoodlad?) want to walk me through.

FWIW I've tested this on a couple of boxes and it works :-)