Bower compatibility for node based projects

My team is utilizing this version of Susy, it actually works great, but we are moving to a node based workflow and would like to be able to pull in this version via bower.
Correct the version to 2.0.0-alpha.4

According to [semver #9](http://semver.org), the prerelease version uses a hyphen.
Merge pull request #239 from stephenway/patch-1

Bower compatibility for node based projects