travis: see if compiler works in build matrix

.travis.yml

@@ -5,13 +5,13 @@ dist: trusty
 matrix:
     include:
         - os: linux
-          env: CC=gcc
+          compiler: gcc
         - os: linux
-          env: CC=clang
+          compiler: clang
         - os: osx
-          env: CC=gcc
+          compiler: gcc
         - os: osx
-          env: CC=clang
+          compiler: clang
 
 cache:
   apt:  true