Remove Gemfile.lock from the gem

"When developing a gem, use the gemspec method in your Gemfile
to avoid duplication. In general, a gem's Gemfile should contain
the Rubygems source and a single gemspec line.

Do not check your Gemfile.lock into version control,
since it enforces precision that does not exist in the gem command,
which is used to install gems in practice.

Even if the precision could be enforced, you wouldn’t want it,
since it would prevent people from using your library
with versions of its dependencies that are different
from the ones you used to develop the gem."

http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/
Merge pull request #505 from vassilevsky/remove-gemfile-lock

Remove Gemfile.lock from the gem