Upgrade to TinkerPop 3.0.0M6.

For reference, the TinkerPop 3 changelog is here:
https://github.com/tinkerpop/tinkerpop3/blob/master/CHANGELOG.asciidoc

The only change from M5 to M6 that affected us was:
' GraphSON format no longer makes use of hiddens JSON key. Its all just properties.'
Add Confirm Traversal API test.

Adds a test that finds a functions of the java GraphTraversal proxy and
compares then against an expected list. This will inform us if the API
changes (not too unlikely, given that the sack API was added in M5), but
more importantly gives us an index of methods that we must provide
wrappers for in order to provide a complete implementation.
Merge pull request #27 from jimlloyd/chore/Upgrade-gremlin-v3-to-use-TinkerPop-3.0M6_83941942_r1

Upgrade gremlin v3 to use TinkerPop 3.0.0M6