Make JsonValueModule compatible with Jackson < 2.8

If an old version of jackson-databind is on the class path (say, 2.5.3) then calls to contentAsObject fail with NoSuchMethodError because JsonValueModule uses JsonParser.currentToken which was added in Jackson 2.8.

Can we rewrite JsonValueModule to be compatible with old Jackson? If not, we need to include it in the pre-flight check for using the non-shadowed Jackson, or perhaps always wrap the JacksonJsonSerializer with JsonValueSerializerWrapper so JsonObject/JsonArray are always (de)serialized using our repackaged Jackson.

Suggested release note text:

contentAsObject() and contentAsArray() no longer fail when a version of Jackson prior to 2.8.0 is on the class path.