Update & loosen moneta dependency

* Updated to use new store init API (as of 0.7.0)
* No need for memcache-client dev dependency - moneta now prefers dalli
Fix `rake spec`
Fixes behaviour when created_at data missing

This closes #6 & #16 which addressed the issue partially.

Specs now use shared example for stores to avoid duplication and ensure that the testing is consistent.
Made NullStore consistent with other stores
Explicitly hide the return value of delete

This is inconsistent between stores so should not be replied upon