Add a source parameter to rbenv::gem

Add an optional `source` parameter to `rbenv::gem`. This lets you specify special (e.g. private) gem repositories.

For example:

``` puppet
rbenv::gem { 'mygem':
  user    => 'larry',
  ruby    => '1.9.3-p392',
  source  => 'http://gems.mybox.es/',
}
```

...will go get the gem `mygem` from the repository at `http://gems.mybox.es/`.