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.
What we have to do is to put a kind of list of OCA repositories to download (account-payment, account-financial-tools, ...), and let the script download them for the correct version?

What do you think?