[TravisCI] bump min osx_image version

.travis.yml

@@ -190,7 +190,7 @@ matrix:
     # osx #
     #######
     - os: osx
-      osx_image: xcode7.3 # to compile binaries that support older versions of Mac, and 10.11 is the min version that brew provides bottles
+      osx_image: xcode8.3 # to compile binaries that support older versions of Mac, and 10.12 is the min version that brew provides bottles
       env:
         - TEST=neko
         - DEPLOY_NIGHTLIES=1