Update test suite and add Gemfile for easier contributing

First off all added development dependencies and a Gemfile, so that setting up the gem for testing would be  as easy as it gets.

Then removed patched MockRedis (it seems it is not needed anymore since "pipelined" isn't used anywhere).

Lastly cleaned up/refactored the specs.

This might be not the cleanest of pull requests so if You see any issues, please let me know.