[Java][RESTEasy] Fix version incompatibility

Created by: delenius

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Description of the PR

The RESTEasy client has incompatible versions for its Jackson libraries, as can be seen in this partial output from mvn dependency:tree in a generated client:

[INFO] +- com.fasterxml.jackson.core:jackson-core:jar:2.6.4:compile
[INFO] +- com.fasterxml.jackson.core:jackson-annotations:jar:2.6.4:compile
[INFO] +- com.fasterxml.jackson.core:jackson-databind:jar:2.6.4:compile
[INFO] +- com.fasterxml.jackson.datatype:jackson-datatype-joda:jar:2.6.4:compile
[INFO] +- joda-time:joda-time:jar:2.9.9:compile
[INFO] +- com.brsanthu:migbase64:jar:2.2:compile
[INFO] +- org.jboss.resteasy:resteasy-jackson2-provider:jar:3.1.3.Final:compile
[INFO] |  \- com.fasterxml.jackson.jaxrs:jackson-jaxrs-json-provider:jar:2.8.6:compile
[INFO] |     +- com.fasterxml.jackson.jaxrs:jackson-jaxrs-base:jar:2.8.6:compile
[INFO] |     \- com.fasterxml.jackson.module:jackson-module-jaxb-annotations:jar:2.8.6:compile
[INFO] +- com.fasterxml.jackson.datatype:jackson-datatype-jsr310:jar:2.6.4:compile
[INFO] +- com.github.joschi.jackson:jackson-datatype-threetenbp:jar:2.6.4:compile

The core library (etc) uses 2.6.4, but resteast-jackson2-provider uses 2.8.6.

This causes a NoSuchMethodError when trying to invoke an API call from the client, in some cases. This PR fixes this by bumping the jackson version up to 2.8.6.

Note that previously, the version for the jackson-datatype-threetenbp library was also set to ${jackson-version}. However, this no longer works, because there is no 2.8.6 version of that library, so I kept that at 2.6.4 (which seems to work).