Fix composer update via HHVM

Hi,

When i try to update my dependencies using hhvm (hhvm /usr/local/bin/composer update), i get an error from your script:

sh: 1: /usr/bin/hhvm --php: not found

This is because the --php argument is escaped with the command, this PR fix it (maybe a better way, but i got no time to pend on it)

Thx
@xabbuh is right. We are using `()` everywhere... well except here apprentely ;)