Update .travis.yml

Author: gmaillet

.travis.yml

@@ -6,6 +6,9 @@ matrix:
     dist: trusty
     sudo: required
     compiler: gcc
+  - os: osx
+    compiler: clang
+
 
 script:
 - mkdir build && cd build && cmake .. && make && cmake -E tar "cfvz" TestCPP-$TRAVIS_OS_NAME.tgz