Disabled Travis build for feature branches

Given that all feature branches have a PR which gets build by Travis, it is useless to build the branches as well.
This rule is to allow us to create maintenance branches for releases in the future and they will be build without having us to remember to enable them.

Hopefully we won't create such branches though. It is really necessary only in case we break BC (keeping a maintenance branch for the previous major version). Otherwise working in master and letting people update to the next minor version (which is BC) is much easier and does not require maintaining several branches (see Twig: no maintenance branch has ever been created in 3 years)