tests: only check Gemfile.lock if it exists

I'm working on packaging gemnasium for Fedora, and we run the rspec tests as part of the build. I found a test failure, and I hope you'll consider accepting this pull request.

The test suite assumes that Gemfile.lock exists. However, the gemnasium gem file on RubyGems.org does not contain a Gemfile.lock. This pull request will cause the test suite to skip the test if the Gemfile.lock file does not exist.