use clang

Author: pcmoritz

.travis.yml

@@ -80,8 +80,7 @@ matrix:
     - ARROW_TRAVIS_JAVA_BUILD_ONLY=1
     # ARROW-2999 Benchmarks are disabled in Travis CI for the time being
     # - ARROW_TRAVIS_PYTHON_BENCHMARKS=1
-    - CC="clang-6.0"
-    - CXX="clang++-6.0"
+    - MATRIX_EVAL="CC=clang-6.0 && CXX=clang++-6.0"
     before_script:
     # (ARROW_CI_CPP_AFFECTED implies ARROW_CI_PYTHON_AFFECTED)
     - if [ $ARROW_CI_PYTHON_AFFECTED != "1" ]; then exit; fi