build: use a different Travis-CI environment.

Travis-CI is so out of date that if you want something as simple as
libsdl2-dev you need to enable sudo and use the huge VM environment
instead of the containers that boot in a few seconds.

.travis.yml

@@ -1,4 +1,5 @@
-sudo: false
+sudo: required
+dist: trusty
 language: c++
 env: VERBOSE=1