Add "[skip ci]" to MantisBT commit messages

Isn't there also the option to change the scheduling for exports from TWN and imports to Git? Projects have the option to define when they occur. May be the current scheduling is troo frequent (twice a week by default) and could be changed (once a week for example, so that you'll also reduce the frequency of automated builds in Travis, if this becomes now too costly).

Using branches for imports in Git IMHO is the best solution if you want to preserve the frequency (and possibly even increase it, e.g. each day) it does not prevent setting up another Git-specific bot if needed to sync that branch to the master, witrh a bot that you can schedule as you want, pause and restart when needed, for example pausing it when you're stabilizing a version of your project for a coming release)