Explicitly use Ruby 1.8.7
Update how we connect to memcache

The `memcached-northscale` gem no longer compiles on Heroku, so we switch to
Heroku's now-recommended way of doing things:
https://devcenter.heroku.com/articles/memcachier#rails-3-and-4 .
Don't load RSpec so we can run Rake in production