[TravisCI] macOS 10.10 is too old according to homebrew

.travis.yml

@@ -141,7 +141,7 @@ matrix:
     # osx #
     #######
     - os: osx
-      osx_image: xcode6.4
+      osx_image: xcode7.3
       env:
         - TEST=neko
         - DEPLOY_NIGHTLIES=1