Really use flickraw-cached.

The way the Gemfile was changed after
4deb03f7f698dfe6a308dc301515a2dffc16bca1 caused flickraw (non-cached)
to be loaded instead. This is one of the two way to straighten this,
the alternative is to just require flickraw. Either seem to work.
flickr: sanitize handling of API keys and secrets.

The simple API key is not enough anymore to fetch Flickr images, the
shared secret is needed as well. Since this is a secret, it can't be
stored in the GIT repository, so instead require a configuration on
the initializer itself.

Possibly, this should be configured at runtime like it's done for
Akismet.
Merge pull request #122 from Flameeyes/master

Flickr fixes and improvements. Tested and specs pass like a charm.