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