Do not install requirements from github.txt in editable mode

I'm not sure what started this pattern, but now it seems that everybody is following it with any additions or changes to github.txt.

@e0d @cpennington @nedbat 
Can you guys think of a reason we'd want/need to install these requirements in editable mode?

References:
http://pip.readthedocs.org/en/latest/reference/pip_install.html#editable-installs
https://pythonhosted.org/setuptools/setuptools.html#development-mode