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
If you get more than one argument, it won't work. So, this method should return an array instead (see my comment on the PR on symfony/symfony).