Move to using .gemspec

[gravitystorm]

When writing gems it's best to avoid using Gemfile.lock and instead put dependencies into a .gemspec file. Further reading at http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/

We need to

• create a gemspec file with all the normal info about the gem
• Add the runtime and development dependencies, perhaps with pessimistic version constraints
• remove Gemfile.lock from the repo
• replace the contents of the Gemfile with the one command gemspec