Add Hound CI to catch css syntax errors

[gemfarmer]

As annoying as hound is, I think it is a really effective way of ensuring that our css code remains clean. A good example of this implementation is the 18F site:

• .hound.yml
• circle.yml
• .scss-lint.yml

The lint file should probably be updated from the 18F/frontend repo:

https://github.com/18F/frontend/blob/master/.scss-lint.yml

[adunkman]

Made obsolete by #19, which runs linting in CI.