update NewtonSoft Json to 6.0.2

[webcoyote]

Updated package config and project files to use Newtonsoft.JSON v6.0.2 instead of 5.0.3

[mroach]

It is a mistake for RollbarSharp.Mvc4Test to be referencing a different version of Json.Net than the main project, however I'd like to leave it at 5.0.3 since the nuget package only requires 5.0 as a minimum version. I wouldn't want to require RollbarSharp users to upgrade to 6.0 unless there's a compelling reason. What do you think?

[webcoyote]

Version 6.0.2 of NewtonSoft Json appears to have a lot of bug-fixes over 5.x.x (listed here https://json.codeplex.com/releases/view/117958 and https://json.codeplex.com/releases/view/121470); while I haven't actually encountered any of those bugs I'm loathe to roll back. In fact I'll probably update my branch to use 6.0.3 shortly since there are even more fixes (https://json.codeplex.com/releases/view/121470).

I'll keep using my fork of RollbarSharp which includes the more recent version of Json, and you can close this pull request without merging if you think that's the right thing to do.

[mroach]

@webcoyote If your project is already using 6.x, does nuget still try to force you to install 5.x?

[webcoyote]

@mroach Nope; when I build it only gets 6.x.