Upgrade to Jackson 2.7

[nbauernfeind]

Upgrades to Jackson 2.7 branch. All current tests pass.

Depends on this databind pull-request:
FasterXML/jackson-databind#1129

[cowtowncoder]

Ok; I went ahead and created 2.6 branch first, and can merge this next. Thank you for tackling the upgrade issues!

One quick question: do you think it would be possible and/or make sense to push 2.7.0 and 2.7.1 versions first? I know they would not address all the issues, but there are many users who expect a full set of patch versions of all modules. Not sure if that is enough of a reason to push versions that are known to have issues tho.

Anyway, I'll merge this first, and I also invited you to be member of jackson-scala team which should have at least write access and be able to make releases. If not, please let me know and I will change settings as necessary.

[nbauernfeind]

The trouble with releasing a 2.7.0 and 2.7.1 is that without the databind patch (issue 1129) that the majority of use cases would simply fail -- anytime someone used a Map it would be deserialized as a Collection of Tuples which would straight up fail. I think it makes more sense to add an "important update" to the readme for the 2.7 branch and only release 2.7.2.

[cowtowncoder]

@nbauernfeind yes, I was afraid this might be the case, but wanted to be sure. Let's plan on 2.7.2 only then.