try prefer source on travis build
try prefer source on travis build

brianium/paratest,chbiel/paratest,localheinz/paratest,ThomasDelteil/paratest,RojerUA/paratest,ThomasDelteil/paratest,quizlet/paratest,brianium/paratest,quizlet/paratest,chbiel/paratest,RojerUA/paratest,localheinz/paratest