Add stdlib version requirement to Modulefile

I had some trouble installing your module using librarian-puppet version 0.9.10. I finally tracked it down to stdlib not having a version in Modulefile. Here's a gist illustrating the problem using the master branch of your repo and my branch that adds a version to stdlib.
https://gist.github.com/bderickson/8815816

I believe I could use the latest version of the librarian-puppet-maestrodev fork of librarian-puppet and things worked fine, but we can't upgrade to that for other reasons.

Version 3.0.0 of stdlib supports Puppet 2.7.x while later versions of stdlib drop that support, so I think specifying the version as I have should be pretty safe since versions of Puppet < 2.7 are EOL.