Add support for Newtonsoft.Json 8

[gmarz]

Released a few days ago https://github.com/JamesNK/Newtonsoft.Json/releases/tag/8.0.1

[Mpdreamz]

I propose we release 1.7.2 as is against json.net 7 then do a NEST1.8 release next week. It be nice if we figure out a way to support both 7 & 8 sanely in both the 1.x and master branches.

Vote to bump to 1.8 👍 /cc @gmarz @russcam

[Mpdreamz]

Another option (perhaps the most sane one) is to internalize our JSON.NET dependency. This might have some implications with peoples custom converters though,

[alinia]

Please bump to 1.8 👍