Fix broken gem configparser dependency

The configparser gem is being used now in the library, but it is only specified in the Gemfile, which is not included when the gem is packaged up. Hence, when I pull the new softlayer_api gem into my project and run `bundle install`, configparser is not installed, and I get a runtime error about this missing dependency from the softlayer_api gem.

All these dependencies should be declared in the .gemspec instead. The Gemfile is usually for projects deployed as apps (or dev deps), rather than libraries like this one.

More information about this at http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/