Fixed flattr URL (fixes #172)

I could not test this because of SASS/Compass dependency hell on my machine :/ :rage: 

@lukasmartinelli can you test this and deploy if it works? :)

PS: There's also https://pypi.python.org/pypi/pyScss/. It doesn't support 100% of what SASS/Compass does, but should mostly work. And you can install it via pip.