Add missing package provider

The current version of this module expects, that user has set `homebrew` as a default package provider elsewhere (e.g. in `site.pp`) while it may not be necessarily true.

If `homebrew` is not set as default `provider`, then the git package installation ends up in following error:

```
Error: Parameter ensure failed on Package[boxen/brews/git]: Provider must have features 'versionable' to set 'ensure' to '2.1.2-boxen1' at /opt/boxen/repo/shared/git/manifests/init.pp:25
Wrapped exception:
Provider must have features 'versionable' to set 'ensure' to '2.1.2-boxen1'
Wrapped exception:
Provider must have features 'versionable' to set 'ensure' to '2.1.2-boxen1'
```
Ah, I didn't know there's an intention to support boxen on Linux or anywhere else except OSX.
I will send fix in a moment though.