Jackson 2 -> 3 Migration

[ryber]

Hello community.
We upgraded Jackson from 2 to 3 in version 4.7.0. This was originally spurred by Spring updating Jackson in their most recent version. The upgrade had no public consequence to Unirest in terms of its own signatures and Jackson is not required for Unirest, so it was made a minor version bump. This was a mistake.

We should have introduced a new modules dedicated to Jackson 3 as its own changes are significant. In order to allow folk to migrate more easily we have re-introduced Jackson 2 as its own module (unirest-modules-jackson-legacy).

Thank you for your attention to this matter.