Also install libffi-dev on travis.
Also install libffi-dev on travis.

Plexxi/st2,Plexxi/st2,nzlosh/st2,Plexxi/st2,nzlosh/st2,nzlosh/st2,StackStorm/st2,StackStorm/st2,StackStorm/st2,Plexxi/st2,nzlosh/st2,StackStorm/st2