fix python3 error caught by travis
fix python3 error caught by travis

Throwing Tasks in a set requires them to be hashable, which was not the
case on python3.

elkingtonmcb/pybuilder,alex-dow/pybuilder,pybuilder/pybuilder,locolupo/pybuilder,esc/pybuilder,arcivanov/pybuilder,onesfreedom/pybuilder,arcivanov/pybuilder,elkingtonmcb/pybuilder,TimYi/pybuilder,pybuilder/pybuilder,Danielweber7624/pybuilder,paolodedios/pybuilder,paolodedios/pybuilder,esc/pybuilder,alex-dow/pybuilder,Danielweber7624/pybuilder,Designist/pybuilder,paolodedios/pybuilder,Designist/pybuilder,esc/pybuilder,locolupo/pybuilder,pybuilder/pybuilder,onesfreedom/pybuilder,arcivanov/pybuilder,TimYi/pybuilder