Force travis to build with clang on MacOS

Summary:
Attempt to force travis to build with clang on MacOS
Closes https://github.com/facebook/rocksdb/pull/2408

Differential Revision: D5186635

Pulled By: yiwu-arbug

fbshipit-source-id: dbb779eff07b1cb7dbd2092631303cf946316656

.travis.yml

@@ -4,6 +4,9 @@ language: cpp
 os:
   - linux
   - osx
+compiler:
+  - clang
+  - gcc
 osx_image: xcode8.3
 jdk:
   - oraclejdk7
@@ -35,6 +38,10 @@ matrix:
     env: TEST_GROUP=2
   - os : osx
     env: JOB_NAME=cmake-mingw
+  - os : linux
+    compiler: clang
+  - os : osx
+    compiler: gcc
 
 # https://docs.travis-ci.com/user/caching/#ccache-cache
 install: