Rubygems is a virtual package on Ubuntu since 15.04

`rubygems` is now a virtual package on Ubuntu since at least 15.04. `rubygems` is now provided by the `ruby` package. This means that the `puppetlabs-ruby` module now will log :

```
Notice: /Stage[main]/Ruby/Package[rubygems]/ensure: ensure changed 'purged' to 'present'
```
for each run.

While this is only a minor problem, it does break the reporting by showing this package as newly installed at each run.