Add alias to run "composer" with hhvm

Via http://vanderveer.be/speed-up-composer-by-using-hhvm-including-a-slowtimer-error-fix/

Gives substantial speed increase:
```
$ time php /usr/local/opt/composer/libexec/composer.phar update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Nothing to install or update
Generating autoload files

real	0m44.593s
user	0m41.770s
sys	0m0.313s
```

```
$ time hhvm /usr/local/opt/composer/libexec/composer.phar update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Nothing to install or update
Generating autoload files

real	0m7.922s
user	0m5.742s
sys	0m0.210s
```