Move Sass monkey patching out of global load path

The method with which we were monkey-patching Sass didn't follow best
practices. Move the patches out of the global path so that `scss-lint`
can be included in other projects.

See http://weblog.rubyonrails.org/2009/9/1/gem-packaging-best-practices
for a full explanation.

Fixes #39.

Change-Id: I666ccfbec8b97e0a04dc6831aae169dfdbde4702
Reviewed-on: https://gerrit.causes.com/30468
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>