Nuget warning for Newtonsoft.Json

[MarkusKgit]

The CloudinaryDotNet.nuspec file is currently pointing to version 12.0 of Newtonsoft.Json, which does not exist. The correct version is 12.0.1.

This leads to the following warning when using the library:
CloudinaryDotNet 1.9.1 depends on Newtonsoft.Json (>= 12.0.0) but Newtonsoft.Json 12.0.0 was not found. An approximate best match of Newtonsoft.Json 12.0.1 was resolved.

This is a similar problem to #116