Run Travis builds for all branches

[pamil]

This prevents me from testing branches on my fork. With Humbug being an organisation, we can all make PRs from our forks, so no duplicated builds will be created. Anyway, it can be configured in Travis project settings (if not already done in .travis.yml as it is now).

[snapshotpl]

I have fixed this in #207

[pamil]

@snapshotpl thanks for that! Nevertheless, your PR is quite more complex than this one and since we got failing test suite on master, I think that we should do the bare minimum to make existing Travis CI config passing before merging other PRs. This one will prevent me from spamming Humbug with builds as the bug mentioned in #219 (comment) will be solved mostly by trial and error :)