chore(gulp): clean before build
chore(gulp): clean before build

Addresses https://github.com/driftyco/ionic-cli/issues/877.

tommy-boy/AMZNDEALS,abdolmajidazad/neveshtak,ericel/weytindey,pierrecabriere/Greencharge,minhlong/NamHoa-Ionic,nReality/SUGSA-ScrumGathering-App2016,nReality/AgileAfricaConf-app,subsolo/ITHC,fmsouza/mangiare,Beslinda/catalogIonicApp,kumarnavinrai/finalmcu,nReality/AgileAfricaConf-app,TobiasSchaeuble/HMAD,driftyco/ionic-conference-app,nReality/SUGSA-ScrumGathering-App2016,laodice/ionic-conference-app-session2,ericel/weytindey,nReality/SUGSA-ScrumGathering-App2016,ericel/weytindey,nReality/AgileAfricaConf-app,nReality/AgileAfricaConf-app,minhlong/NamHoa-Ionic,Beslinda/catalogIonicApp,TobiasSchaeuble/HMAD,abdolmajidazad/neveshtak,subsolo/ITHC,francarmona/ionice-contryside-events-app,minhlong/NamHoa-Ionic,nReality/SUGSA-ScrumGathering-App2016,nReality/AgileAfricaConf-app,nReality/SUGSA-ScrumGathering-App2016,nReality/AgileAfricaConf-app,francarmona/ionice-contryside-events-app,laodice/ionic-conference-app-session2,nReality/SUGSA-ScrumGathering-App2016,driftyco/ionic-conference-app,fmsouza/mangiare,nReality/SUGSA-ScrumGathering-App2016,TobiasSchaeuble/HMAD,laodice/ionic-conference-app-session2,sinholic/aero-flyer,fmsouza/mangiare,abdolmajidazad/neveshtak,kumarnavinrai/finalmcu,sinholic/aero-flyer,subsolo/ITHC,Beslinda/catalogIonicApp,nReality/AgileAfricaConf-app,francarmona/ionice-contryside-events-app,driftyco/ionic-conference-app,pierrecabriere/Greencharge,pierrecabriere/Greencharge,kumarnavinrai/finalmcu,tommy-boy/AMZNDEALS,tommy-boy/AMZNDEALS,sinholic/aero-flyer,fmsouza/mangiare