Allow enums to be created without associated indexes
Allow enums to be created without associated indexes

As far as I can tell, currently the `_index` option for enums doesn't do anything so I'm using it to suppress the `Neo4j::ModelSchema.add_required_index` exception.
(cherry picked from commit f4599d4524751d69090c464f171a748d7e3d504a)

neo4jrb/neo4j,neo4jrb/neo4j,leviwilson/neo4j,cheerfulstoic/neo4j,cheerfulstoic/neo4j,leviwilson/neo4j