Allow rspec-rails to load without Rubygems loaded

Rspec-rails is a great gem citizen, and doesn't explicitly require Rubygems anywhere. However, while working on a Rails app that doesn't require Rubygems before loading Rails, we've noticed that rspec-rails expects Rubygems to be loaded even though it doesn't require it.

This pull request simply checks for Rubygems before calling Rubygems methods, so that rspec-rails works without Rubygems already being loaded.