Fatal error if taxonomy module is not enabled

This module depends on the taxonomy module, although it is not declared as a dependency in the modules page. Going to the "Add Featured Content Block" page without the taxonomy module enabled results in a PHP fatal error ("Call to undefined function taxonomy_get_vocabularies()"). Enabling the taxonomy module fixes this error, making it clear that this is a dependency problem. Note that this is not the same as #1181214: Call to undefined function taxonomy_node_get_terms_by_vocabulary(), as that occurs when the module is enabled.