Fix occasionally failing travis builds

We require sudo right now, tell this travis, so that they don't try to
run it on their container based infrastructure.

.travis.yml

@@ -8,6 +8,7 @@ env:
   - PLATFORM=Linux
 before_install: ./util/travis/before_install.sh
 script: ./util/travis/script.sh
+sudo: required
 notifications:
   email: false
 matrix: