OGM-622 Upgrade Neo4j version to 2.1.5

https://hibernate.atlassian.net/browse/OGM-622

At the moment `org.neo4j:neo4j` includes different version of the same libraries. While this is not usually a problem because maven will automagically select only one to resolve conflicts this is a problme for because we use the enforce plugin.

That's the reason in the BOM I exclude some dependencies from neo4 to include them with the rgiht version.

I've talked with the Neo4j folks and they agreed to apply a patch if I send it, I will do that soon.
I'm still not sure (maybe I forgot) why we need two versions in the first place. Can't we use 2.1.3 for everything? Worth adding a comment (and be it only so I don't have to ask again in a few weeks from now ;).