Make Dalli::Client server more flexible

Before it was only possible to use the host specified in `bandit.yml` (and
the port was ignored as well). Now, if the host isn't specified in the
config Dalli will properly fall back to using `ENV["MEMCACHE_SERVERS"]` or
`127.0.0.1:11211`.

See: https://github.com/mperham/dalli/blob/master/lib/dalli/client.rb#L29

This makes it easier to get it running on platforms like Heroku.