Switch builds to use Trusty.

SDL2 packages are too new for Ubuntu Precise, which is from 2012.
Switch to the Trusty beta environment, as documented here:

https://docs.travis-ci.com/user/trusty-ci-environment/

.travis.yml

@@ -2,6 +2,11 @@ language: c
 
 compiler: gcc
 
+# Travis uses Ubuntu 12.04 (Precise) for builds by default, which is too
+# old and missing the SDL2 packages, so use Trusty instead.
+sudo: required
+dist: trusty
+
 # TODO: Remove old SDL library dependencies once sdl2-branch is merged.
 addons:
     apt: