travis: test against our minimum versions
req: create min-required.txt in complex version case

E.g. properly strip:
>=1.0,<1.6 --> ==1.0
>=1.0,!=1.0.1 --> ==1.0
req: export non-versioned requirements to min-required.txt
travis: show what is installed in the virtualenv
docs: Bump minimum Sphinx version to 1.2.0

Earlier versions either don't work or fail on some of our docstrings.