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
The problem is that you might still need to escape the PHP path. That's a tricky issue as you cannot make a difference between what should be esacped and what should not.

I think the problem is more in `PhpExecutableFinder` where we can return some options like `--php`.