travis: Update to the Ubuntu Trusty image.

This will hopefully fix wget from x.org (no real reason explained in
Travis CI bug reports), and may also mean that we can enable LLVM driver
builds.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Rhys Kidd <rhyskidd@gmail.com>

.travis.yml

@@ -1,6 +1,7 @@
 language: c
 
-sudo: false
+sudo: true
+dist: trusty
 
 cache:
   directories: