updated activesuport and added fakeredis

Hi Schubert,

I just was wondering why bundler won't update the redis_analytics gem. By forcing it into updating (No bundlers were harmed in this process, I promise!) I got the message that there is no compatible version because `activesupport 3.2.0` is still a dependency.  So I changed it to `'activesupport', '~> 4.0.2'`

A question that just bubbled up in my head while I am writing this: Is redis_analytics using anything from activesupport? I know it used `ActiveSupport::Concern` back in the days when I started the cleanup, but besides the one `require 'active_support/core_ext'`line in lib/redis_analytics/redis_analytics.rb#L6 it is not used anywhere. (Maybe activesupport could be removed from the dependencies?) 

Another change I've just made: I don't like spinning up an redis-server when I run the specs on my machine. I think it's much better to use an library that mocks an redis-server, which `fakeredis` does. So I added the gem to the development group, so you don't have to run redis-server.