[TravisCI] move osx_image to matrix

.travis.yml

@@ -41,7 +41,6 @@ env:
 
 sudo: required
 dist: trusty
-osx_image: xcode6.4
 addons: &addons
   ssh_known_hosts:
     - haxe.org
@@ -213,13 +212,14 @@ matrix:
     # osx #
     #######
     - os: osx
+      osx_image: xcode6.4 # to compile binaries that support older versions of Mac
       env:
         - TEST=macro,neko,js,php,python,hl,lua
         - DEPLOY_NIGHTLIES=1
       install: *install_osx
 
     - os: osx
-      osx_image: xcode8.3
+      osx_image: xcode8.3 # to compile C++ faster
       env:
         - TEST=cpp
         - HXCPP_COMPILE_CACHE=~/hxcache