Travis: clang 3.5.0 on Linux

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

all trusty images have sudo, and actually it is required
to use this new Travis environment per docs. This makes
it actually do it. Clang is 3.5 now.

.travis.yml

@@ -1,6 +1,6 @@
 language: cpp
 dist: trusty
-sudo: false
+sudo: required
 
 matrix:
   include: