Remove Berksfile.lock

Having these included in the repo can potentially cause conflicts, for example the Opscode nginx cookbook depends on [`apt "~> 2.2"`](https://github.com/opscode-cookbooks/nginx/blob/master/metadata.rb#L11), which conflicts with the `locked_version` of `apt` in the `Berksfile.lock` file.

We were having trouble locally when creating a wrapper cookbook and using test-kitchen to test, because of the version conflicts.