Use trusty dist; we seem to be trying to compile pypy for some reason...

Use trusty dist; we seem to be trying to compile pypy for some reason otherwise. Maybe this will fix it. [skip appveyor]

.travis.yml

 # .travis.yml based on https://github.com/DRMacIver/hypothesis/blob/master/.travis.yml
 language: c
 sudo: false
+dist: trusty
 
 env:
     global: