Isolated module tests

Alternate implementation for #16, but using TravisCI own isolated test environments.
Built on top of #29 - review commits from 3eb32a2 	"Allow envs testing specific modules" onward.
It would be easy to add, but I don't see an advantage on having two different ways to do the same thing. 
Won't it be neater to install those deps under HOME? Current branch under tests is the only thing contained in TRAVIS_BUILD_DIR, so that's the place to target Coverage and similar checkers ...