Be compliant with the Puppet Forge

This fixes #143, as reported by @ryanycoleman

The ~> dependency expression used here is not expressly supported by the
Puppet Forge and module tool. It may not always work as desired.

The currently supported expressions are documented here[1].

[1]: http://docs.puppetlabs.com/puppet/latest/reference/modules_publishing.html#dependencies-in-the-modulefile