Remove log output when redis isn't loaded

[garethrees]

d3cfc5a76 introduced a warn statement that prints when redis is not available.

This gets printed whenever the rails environment gets loaded (so, rails s, rails c,rake`, etc) which is a bit annoying. The output also broke some specs in a project that was checking the stdout of some of our scripts.

@pboling suggested there's a better implementation in http://git.io/vfGPX

[garethrees]

Cheers! Would be great if you could make a new point release on rubygems

[pboling]

@garethrees Released into the series of tubes. I also added a HEAD section to the change log. Feel free to add to it in PRs. :+1:

[garethrees]

:cake: Thanks! Much appreciated.