Make installing dalli and redis gem optional

[doits]

fixes #3 and removes a deprecation warning for Dalli >= 3.0.0

Basically it blows up when doing Suo::Client::Memcached.new or Suo::Client::Redis.new without the respective gem and not passing in a custom client.

I added the check for the custom client in case somebody uses a special client without the official dalli or redis gem.

[doits]

I think the PR is ready now. What do you think?

If you merge and release this, it should be a bigger version change (maybe even 1.0.0??).

[mieko]

@doits Thanks for this branch. It's been working great for me.