Do not assume Rails deployments use memcached

The recipes, templates, etc for memcached should be in a separate cookbook, so
that users may choose what caching backend they want. It is more common to see
Redis anyway, so why someone would choose memcached as the default method of
caching is odd.
Upgrade the default Rails version to something more recent
Increase visibility of code blocks
Include a check for the postgresql gem as a potential database adapter