Fix build

Author: kirillgarbar

paket.dependencies

@@ -9,7 +9,7 @@ nuget YoloDev.Expecto.TestSdk 0.13.3
 nuget Microsoft.NET.Test.Sdk 16.8.0
 nuget altcover ~> 7
 
-nuget System.Collections.Immutable 8.0
+nuget System.Collections.Immutable 7.0
 nuget ExtraConstraints.Fody 1.14.0
 nuget Microsoft.Build.Framework 16.10
 nuget System.CodeDom >= 7.0
@@ -59,4 +59,4 @@ group Docs
 group Analyzers
   source https://www.nuget.org/api/v2
   source https://api.nuget.org/v3/index.json
-  nuget BinaryDefense.FSharp.Analyzers.Hashing 0.2.2
+  nuget BinaryDefense.FSharp.Analyzers.Hashing 0.2.2