switch travis to xenial
switch travis to xenial

xenial has a new enough git (v2.21.0) so don't upgrade.

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