Use RedisCleaner, not RedisKeyCleaner

Initializing with RedisKeyCleaner results in:
`uninitialized constant RedisKeyCleaner (NameError)`
Merge pull request #3 from skymob/fix_class_name_in_readme

Thanks ! Merging now. The tests fail prolly because of outdated RSpec usage.

In any case, hope you didn't have to use this gem because of a bug ;)