Enable travis-ci OSX builds
Enable travis-ci OSX builds

travis-ci does not provide OSX builds for free, except on request.
However if we add it, Wikimedia can request it for the merged
commits, and other developers will notice if it ever becomes available
more widely.

Change-Id: I785019ca5ce806070c0d0c881db93184d8f67b49

wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core,happy5214/pywikibot-core,darthbhyrava/pywikibot-local,hasteur/g13bot_tools_new,jayvdb/pywikibot-core,icyflame/batman,TridevGuha/pywikibot-core,magul/pywikibot-core,hasteur/g13bot_tools_new,h4ck3rm1k3/pywikibot-core,smalyshev/pywikibot-core,VcamX/pywikibot-core,h4ck3rm1k3/pywikibot-core,Darkdadaah/pywikibot-core,xZise/pywikibot-core,npdoty/pywikibot,wikimedia/pywikibot-core,happy5214/pywikibot-core,Darkdadaah/pywikibot-core,hasteur/g13bot_tools_new,emijrp/pywikibot-core,npdoty/pywikibot,magul/pywikibot-core,trishnaguha/pywikibot-core,valhallasw/pywikibot-core,jayvdb/pywikibot-core