bump min mac version to 10.11 such that we can install bottles in Travis

.travis.yml

@@ -212,7 +212,7 @@ matrix:
     # osx #
     #######
     - os: osx
-      osx_image: xcode6.4 # to compile binaries that support older versions of Mac
+      osx_image: xcode7.3 # to compile binaries that support older versions of Mac, and 10.11 is the min version that brew provides bottles
       env:
         - TEST=macro,neko,js,php,python,hl,lua
         - DEPLOY_NIGHTLIES=1