Raise minimum required PS version of PS 6 to 6.1.0 due to 6.0 running out of support now and update Newtonsoft.Json to 11.0.2

[bergmeister]

PR Summary

• Fixes Raise minimum version of PowerShell Core from 6.0.2 to 6.1.0 or 6.1.3 #1159 by raising the minimum required PowerShell version of PS6 to be 6.1 (6.0 is running out of support at some point in March 2019).
• Fixes Upgrade Json.NET version? #1058 by raising the version of Newtonsoft.Json to 11.0.2, which is the version used by PS 6.1.0, see here (the version cannot be higher due to the missing assembly redirect feature in .Net Core and PowerShell Core at the moment)

PR Checklist

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• Summarized changes
• Change is not breaking Well, sort of is, but by design :-)
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
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