Allow for building docs with `make html`

This commit makes use of the virtual env in the `docs` target.
This way, all users have to do to build the documentation is run
`make docs`.

Change-Id: I5c4855d9d0a3fbbfeea53c568f3d9dc6de10ddd5
Tuleap: #636

Makefile

 .PHONY: docs
 docs: env
-	make -C docs html
+	env/bin/sphinx-build -b html docs/source docs/build/html
 
 env:
 	virtualenv env