If mongoid isn't installed, cachetastic dies even if mongoid adapter isn't in use

Great work on cachetastic!

I just want to use a simple memory cache, but as of now, I have to install mongoid as well, because cachetastic dies with this stacktrace:

/Users/trhodes/.rvm/gems/ruby-1.9.3-p385@ha-glue/gems/cachetastic-3.7.0/lib/cachetastic/adapters/mongoid.rb:6:in `<class:Store>': uninitialized constant Mongoid (NameError)
    from /Users/trhodes/.rvm/gems/ruby-1.9.3-p385@ha-glue/gems/cachetastic-3.7.0/lib/cachetastic/adapters/mongoid.rb:5:in `<class:Mongoid>'
    from /Users/trhodes/.rvm/gems/ruby-1.9.3-p385@ha-glue/gems/cachetastic-3.7.0/lib/cachetastic/adapters/mongoid.rb:3:in `<module:Adapters>'
    from /Users/trhodes/.rvm/gems/ruby-1.9.3-p385@ha-glue/gems/cachetastic-3.7.0/lib/cachetastic/adapters/mongoid.rb:2:in `<module:Cachetastic>'
    from /Users/trhodes/.rvm/gems/ruby-1.9.3-p385@ha-glue/gems/cachetastic-3.7.0/lib/cachetastic/adapters/mongoid.rb:1:in `<top (required)>'
    from /Users/trhodes/.rvm/gems/ruby-1.9.3-p385@ha-glue/gems/cachetastic-3.7.0/lib/cachetastic.rb:29:in `require'
    from /Users/trhodes/.rvm/gems/ruby-1.9.3-p385@ha-glue/gems/cachetastic-3.7.0/lib/cachetastic.rb:29:in `block in <top (required)>'
    from /Users/trhodes/.rvm/gems/ruby-1.9.3-p385@ha-glue/gems/cachetastic-3.7.0/lib/cachetastic.rb:28:in `each'
    from /Users/trhodes/.rvm/gems/ruby-1.9.3-p385@ha-glue/gems/cachetastic-3.7.0/lib/cachetastic.rb:28:in `<top (required)>'
...
Mongoid 3 support is unavailable. To use Mongoid 3 do `gem install mongoid`
...

Looks like it could be fixed by adding an if defined?(::Mongoid) with an else body overriding method_missing to do nothing. Not a ruby pro, so don't know if that is the "Right Way (tm)".

I was going to write it up myself, but when I got to testing, a lot of other tests fail. Some from servers not being up, some for other reasons.

Thanks! Let me know if you need any more info!

Start with lib/cachetastic.rb and the required adapter files, especially lib/cachetastic/adapters/mongoid.rb, then reproduce loading the memory cache without Mongoid installed. Review the existing test suite and its reported failures. Done means Cachetastic can use the memory adapter without requiring Mongoid, with relevant tests covering that case.