Be compliant with the Puppet Forge

Like in https://github.com/theforeman/puppet-puppet/commit/27c73a6ab9b7842fa3827ad7bc24fddbdd91c73a.

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