Remove and ignore Gemfile.lock

I recently ran into some parallel computing issues with the clockwork gem. After some research we decided to use your gem to solve our problems by leaning on delayed jobs (which we already have in the app).

I came upon a few things that I thought would be worth adjusting in the gem. I hope you don't mind! Of course, I'm happy to hear your feedback and make any changes necessary to meet your liking :+1: 

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Generally the `Gemfile.lock` should not be included with library code. This allows each developer to carry their own set of locked dependencies and helps ensure that the dependencies of the library are defined in a flexible way.

For more information http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile

What do you think? :blush: