Added missing autoStartTransaction() in getEdges

After running into #489 I looked at the changeset of #465.

It seems that the call `this.rawGraph.index().getRelationshipAutoIndexer().getAutoIndex().get(key, value)` in [Neo4j2Graph.java line 394](https://github.com/tinkerpop/blueprints/blob/master/blueprints-neo4j2-graph/src/main/java/com/tinkerpop/blueprints/impls/neo4j2/Neo4j2Graph.java#L394) caused the `NotInTransactionException` exception.

I added a call to `autoStartTransaction` which fixed the problem for me.

Unfortunately I could not get the test to fail, because it seems that the Indexer is not enabled while executing the tests. Is there any way to enable indexing just for one test?