travis: Add macox/clang test

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

.travis.yml

@@ -14,6 +14,12 @@ matrix:
       env:
         - MATRIX_EVAL="CC=gcc-7 && CXX=g++-7"
 
+    - os: osx
+      osx_image: xcode10
+      compiler: clang
+      env:
+        - MATRIX_EVAL="CC=clang && CXX=clang++"
+
 before_install:
   # This is necessary to solve https://github.com/travis-ci/travis-ci/issues/9649
   - eval "${MATRIX_EVAL}"