Enable mongodb on Travis CI.
Enable mongodb on Travis CI.

We need it for the new register resources check.

pidah/st2contrib,armab/st2contrib,psychopenguin/st2contrib,pearsontechnology/st2contrib,psychopenguin/st2contrib,StackStorm/st2contrib,pearsontechnology/st2contrib,armab/st2contrib,StackStorm/st2contrib,pearsontechnology/st2contrib,armab/st2contrib,pidah/st2contrib,pidah/st2contrib,pearsontechnology/st2contrib,StackStorm/st2contrib,tonybaloney/st2contrib,tonybaloney/st2contrib,tonybaloney/st2contrib