Be compliant with the Puppet Forge

Like in puppet-puppet[1].

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[2].

[1]: https://github.com/theforeman/puppet-puppet/commit/27c73a6ab9b7842fa3827ad7bc24fddbdd91c73a.
[2]: http://docs.puppetlabs.com/puppet/latest/reference/modules_publishing.html#dependencies-in-the-modulefile