Update to use modern rspec 2.14 syntax.

This updates from mocha .stubs to allow(x).to receive(x) syntax,
and tweaks the Gemfile/Rakefile too.
Handle the change to Puppet::Provider::Config in 3.4.
Merge pull request #275 from apenney/fix-rspec

Fix rspec on Puppet 3.4/rspec 2.14.