Fix compatibility problems with Drupal 8.2.x

Currently the module doesn't appear to work with core 8.2.x, it throws errors like this:

Doctrine\Common\Annotations\AnnotationException: [Semantical Error] Couldn't find constant label, class Drupal\metatag_open_graph\Plugin\metatag\Tag\OgImageWidthX. in Doctrine\Common\Annotations\AnnotationException::semanticalError() (line 54 of vendor/doctrine/annotations/lib/Doctrine/Common/Annotations/AnnotationException.php).

Work through all of the problems and ensure the module works correctly.