Let Travis build with both GCC and clang, take 2

Last approach didn't work, hopefully this will work better.

.travis.yml

@@ -3,9 +3,11 @@ matrix:
     - os: linux
       dist: trusty
       sudo: required
-      compiler: 
+      compiler: gcc
-        - gcc
+    - os: linux
-        - clang
+      dist: trusty
+      sudo: required
+      compiler: clang
     - os: osx
       osx_image: xcode7.3
       compiler: clang