Better dependency management

This includes following changes about dependency management:

* Let Bundler refer gemspec file
* Remove `Gemfile.lock` from git repository
These are recommended by the author of Bundler.
http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/

* Specify version of dependency gems
This is to avoid Rubocop not working when new major version of any depedency is released, according to Semantic Versioning:
http://semver.org/