Dependencies improvement

[fphilipe]

Hey there. I was trying to get this up and running outside of rack/rails and hit the following problem. Apparently your code uses Librato.group which is nowhere to be found. Then I read in the README that one is supposed to include either librato-rack or librato-rails by hand. So this Librato.group must be defined in one of those soft dependencies.

It turns out that librato-rack defines this and that librato-rails depends upon librato-rack. Thus it would be a good thing for this gem to depend on librato-rack directly. This will make it work out of the box in plain ruby.

[jaredbrown]

I just ran into this. Thanks for pointing this out.