Specify rake and rspec versions in gemspec

Since the development dependency versions are currently undefined, running `bundle` just grabs the latest of everything. But the specs are failing under RSpec 3. Rather than fixing the tests (which I guess would be a better solution), this just locks the rspec dependency to `2.14.1`, which was the version which Travis was running when [the last successful build] (https://travis-ci.org/resque/redis-namespace/builds/14971572) occurred.