Default to GraphSON 2.0

[Stephen Mallette <sp...@gmail.com>]

I just pushed https://issues.apache.org/jira/browse/TINKERPOP-1414 onto
TINKERPOP-1565 branch as it was related. Note that this work is targeted
for 3.3.0.

Basically, this change means that Gremlin Server and TinkerGraph both
default to GraphSON 2.0 for 3.3.0. It's a breaking change only if you
happened to still use "application/json" as your serialization method. I
suspect that this break would affect REST API users the most, but it's
relatively simple to re-configure Gremlin Server to work the old way. It's
a one line change in gremlin-server.yaml which is written clearly in the
upgrade docs.