Add xcode7 build to make sure that we are not affected by std::make_shared bug

.travis.yml

@@ -9,6 +9,9 @@ matrix:
     - os: osx
       osx_image: xcode7.3
       compiler: clang
+    - os: osx
+      osx_image: xcode7
+      compiler: clang
 sudo: false
 addons:
   apt: