add premailer to requirements

Running tests after `pip install -r requirements.txt` threw ImportError premailer module not found. I think the problem is this line: `-e git+git@github.com:tthieman/dagobah@8030625e37799f14fe1f61eae1b3b738693125e9#egg=dagobah-dev` ...which, incidentally, installed the entire dagobah repo into the virtualenv. Is that necessary?