fix requirements for ubuntu 15.x
fix requirements for ubuntu 15.x

aginzberg/crowdsource-platform,crowdresearch/crowdsource-platform,crowdresearch/daemo,crowdresearch/daemo,shirishgoyal/crowdsource-platform,shirishgoyal/crowdsource-platform,crowdresearch/daemo,aginzberg/crowdsource-platform,crowdresearch/daemo,shirishgoyal/crowdsource-platform,aginzberg/crowdsource-platform,aginzberg/crowdsource-platform,crowdresearch/crowdsource-platform,crowdresearch/crowdsource-platform,shirishgoyal/crowdsource-platform,crowdresearch/crowdsource-platform