Remove PHP 5.3 support.

5.3 is now [EOL] and there is no reason for us to continue to support
this version from this moment on.  Users needing PHP 5.3 support can use
the previous version.

[EOL]: http://php.net/archive/2014.php#id2014-08-14-1
Merge pull request #17 from nubs/php-53-eol

Remove PHP 5.3 support.