Re-enable Travis build auto-cancelling

[wilzbach]

On force-pushes previous builds aren't cancelled:

So imho we should re-enable our working Travis PR canceller for dmd and possibly vibe.d.

If you look at the build history, I manually cancelled these builds.

See also: #75

[MartinNowak]

Was already wondering what was going on, annoying that their auto-cancellation doesn't work for that case.

[CyberShadow]

We don't use Travis any more, do we? Should this be closed?