Remove Gemfile.lock

Gemfile.lock will never use the current pliny version.
It will always reference the previous release.

See https://github.com/interagent/pliny-template/issues/143#issuecomment-60575521

Unanswered questions:

* [ ] How to avoid committing Gemfile.lock?
  * Adding it to .gitignore is not feasible, as this would be then also valid for the project recently created
  * Idea: Use Thor in Creator and rename current .gitignore to .gitignore.tt, and add a new .gitignore ignoring Gemfile.lock
* [ ] Do we need to lock any gem in the Gemfile?
  Pliny must then always support the latest version of each gem...