Problme: OSX build does not use clang

Default compiler on OSX is clang so it should be default.
Solution: switch from gcc to clang for OSX build.

.travis.yml

@@ -22,6 +22,15 @@ env:
 
 matrix:
   include:
+    ##########################################################
+    # Clang on OSX
+    # Travis takes longer to start OSX instances,
+    # so leaving it first for the overall build to be faster.
+    ##########################################################
+    - os: osx
+      osx_image: xcode9.1
+      env:
+        - MATRIX_EVAL="brew install cmake" ZMQ_VERSION=4.2.5 DRAFT=1
   # - env: BUILD_TYPE=cmake DO_CLANG_FORMAT_CHECK=1 CLANG_FORMAT=/usr/local/clang-5.0.0/bin/clang-format
     # os: linux
     # addons:
@@ -39,10 +48,6 @@ matrix:
             - g++-7
       env:
         - MATRIX_EVAL="CC=gcc-7 && CXX=g++-7" ZMQ_VERSION=4.2.5 DRAFT=1
-    - os: osx
-      osx_image: xcode8
-      env:
-        - MATRIX_EVAL="brew install cmake gcc && CC=gcc-7 && CXX=g++-7" ZMQ_VERSION=4.2.5 DRAFT=1
 
 sudo: required
 
@@ -52,6 +57,7 @@ before_install:
 # Build and check this project
 script:
   - eval "${MATRIX_EVAL}"
+  - cmake --version
   - ./ci_build.sh
 
 after_success: