Use an actual memcached server and active support cache store in tests.

There is no reason to use this in memory mock cache adaptor. The only gotcha I
found is that objects with expectations set on cannot be marshaled, they will
contain a reference to a mocha singleton and generate a type error.

@hornairs @boourns @jduff, ok to merge?
Adding gemspec changes
More reasonable gemspec
Merge pull request #15 from Shopify/use_real_cache_on_testing

Use an actual memcached server and active support cache store in tests.