Remove jeweler, rake and rspec from runtime dependencies.

Issue here: https://github.com/whoahbot/dm-redis-adapter/issues/56

Currently, anyone using this gem is forced to install Jeweler, Rake and RSpec, which aren't actually runtime dependencies (unless I'm missing something!). They each carry their own dependencies, so this amounts to about 10 gems that must be installed unnecessarily in order just to use this one.