remove Gemfile.lock from gemspec

Hi,
   The current version have following warning when try to add bundle from git

If you need to use this package without installing it from a gem repository, please contact ilpuccio.febo@gmail.com and ask them to modify their .gemspec so it can work with `gem build`.
The validation message from Rubygems was:
  ["Gemfile.lock"] are not files

Remove Gemfile.lock from gemspec is help.

Best,
-Jirapong