Allow rspec beta versions to match

Currently, rspec beta versions like `rspec-core-3.0.0.beta2` don't match the first regex for rspec version detection and fall through to the second. The second, however, will cause recent rspec versions to prompt for user input and hang vim as a result.

This pull request removes the requirement that the version is last in the string, which allows `rspec-core-3.0.0.beta2` to match appropriately.