Revert "Do not install requirements from github.txt in editable mode"

This reverts commit 61cf1d6edb4609094ba6b84c87a29ff5bec4ee2c.

Conflicts:
	requirements/edx/github.txt
Merge pull request #6121 from edx/clytwynec/revert-not-editable-mode

Revert "Do not install requirements from github.txt in editable mode"