Add hhvm-nightly as a testing environment on Travis CI.
Add hhvm-nightly as a testing environment on Travis CI.

RudyJessop/predis,gopalindians/predis,nguyenthaihan/predis,protomouse/predis,moria/predis,lvbaosong/predis,SwelenFrance/predis,CloudSide/predis,mrkeng/predis,gencer/predis,zhangyancoder/predis,nrk/predis,vend/predis,WalterShe/predis,SecureCloud-biz/predis,dzung2t/predis,quangnguyen90/predis