Rem: PHP 5.3 Support

PHP 5.3 was causing tests to fail due to lack of callable typehint.

Removing PHP 5.3 support across the whole library seems sensible as
it's near EOL anyway.
Merge branch 'feature/rem-php-5.3-support' into develop

* feature/rem-php-5.3-support:
  Rem: PHP 5.3 Support