Add version number to imagick extension

Summary: fixes facebook/hhvm#3969

Went for '3.2.0b2' based on http://pecl.php.net/package-changelog.php?package=imagick:

- we have all those constants (3.2.0b2)
- Imagick implements Countable (3.2.0b1)
- we implement __clone, not clone() (deprecated in 3.1, removed in 3.2)

Reviewed By: @JoelMarcey

Differential Revision: D1694086

Signature: t1:1694086:1416504185:9b07e213bfa49e22dd196970e8ec0474c8a56d39