[.travis] Use composer for HHVM
[.travis] Use composer for HHVM

This fixes the HHVM build failing because pear doesn't exist in HHVM.

Riduidel/rss-bridge,RSS-Bridge/rss-bridge,RSS-Bridge/rss-bridge,Frenzie/rss-bridge,Riduidel/rss-bridge,simonbru/rss-bridge,simonbru/rss-bridge,RSS-Bridge/rss-bridge,kranack/rss-bridge,teromene/rss-bridge,ORelio/rss-bridge,ORelio/rss-bridge,ORelio/rss-bridge,kranack/rss-bridge,kranack/rss-bridge,kranack/rss-bridge,sebsauvage/rss-bridge,teromene/rss-bridge,Riduidel/rss-bridge,Frenzie/rss-bridge