Delete and .gitignore the Gemfile.lock

Per http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/

This additionally solves an issue with running the tests. The version of RSpec specified in the lockfile is 2.5.0, which pre-dates the addition of [RSpec::Core::SharedExampleGroup#shared_examples](http://rdoc.info/github/rspec/rspec-core/RSpec/Core/SharedExampleGroup:shared_examples), which is used in [spec/bcrypt/error_spec.rb](https://github.com/codahale/bcrypt-ruby/blob/master/spec/bcrypt/error_spec.rb#L5-L15), which threw a `NoMethodError` during a test run.