Set nightly to allowed_failure
Set nightly to allowed_failure

travis-ci 'nightly' recently became Python 3.6, causing
two problems which are being investigated.
travis-ci will be providing a proper Python 3.5 target soon.

Bug: T106224
Change-Id: I9822e25a05f2e2d2a95fb9d53cb55cbeadf11f00

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