Fixed composer requirements for PHP55

There was an issue in which composer was refusing to install with a PHP55 installation.

**Before:**

	$ brew install composer 
	composer: Composer PHP requirements check has failed. The following verification command:

	    curl -s http://getcomposer.org/installer | /usr/bin/env php -d allow_url_fopen=On -d detect_unicode=Off -d date.timezone=UTC -- --check

	failed because of the following reason:

	    Some settings on your machine may cause stability issues with Composer.
	    If you encounter issues, try to change the following:
    
	    The apc.enable_cli setting is incorrect.
	    Add the following to the end of your `php.ini`:
	        apc.enable_cli = Off
    
	    The php.ini used by your command-line PHP is: /usr/local/etc/php/5.5/php.ini
	    If you can not modify the ini file, you can also run `php -d option=value` to modify ini values on the fly. You can use -d multiple times.
    

	composer: Missing PHP53 or PHP54 from homebrew-php. Please install one or the other before continuing
	Error: Unsatisifed requirements failed this build.

**After:**

	$ brew install composer
	==> Downloading http://getcomposer.org/download/1.0.0-alpha7/composer.phar
	######################################################################## 100.0%
	==> Caveats
	Verify your installation by running:
	  "composer --version".

	You can read more about composer and packagist by running:
	  "brew home composer".
	==> Summary
	(beer)  /usr/local/Cellar/composer/1.0.0-alpha7: 3 files, 796K, built in 2 seconds