NCBC-398: Support Json.NET >= 6.0.1

This commit removes restriction on Json.NET version and bumps the build
version from Nuget Version 4.5.11 to 5.0.8 (assembly versions are slightly
different). 5.0.8 is the last released version before 6.0.1.

Change-Id: I446d22f790862ffc45afd6e1e27c32d52e166a7a
Reviewed-on: http://review.couchbase.org/33938
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>