Update Newtonsoft.Json to v11

[zplan]

The cake version 0.27.2 has updated the Newtonsoft.Json reference from v9.0.1 to v11.0.2.

Since Cake.Json still using v9 from Newtonsoft.Json, there is now a warning on every start up about the different Json versions:
(2843,12): warning CS1701: Assuming assembly reference 'Newtonsoft.Json, Version=9.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed' used by 'Cake.Json' matches identity 'Newtonsoft.Json, Version=11.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed' of 'Newtonsoft.Json', you may need to supply runtime policy

Update Cake.Json to use v11 from Newtonsoft.Json