Test sage 8.3 on macOS 10.11.6
Test sage 8.3 on macOS 10.11.6

On later versions of macOS on Travis it seems that sage cannot find some gcc headers.
cswiercz/abelfunctions,abelfunctions/abelfunctions,abelfunctions/abelfunctions,cswiercz/abelfunctions,abelfunctions/abelfunctions,cswiercz/abelfunctions