Fixes for PHP 5.3 and 5.4 stemmer extensions

The `php53-stemmer.rb` and `php54-stemmer.rb` formulae were both using the `AbstractPhp55Extension` class as a base class, which created a dependency to PHP 5.5, which does not seem correct.

Changed each file to the seemingly proper base class, and then the extension installed fine for PHP 5.4.