Switch to stable version of .NET framework reference assemblies (#348)

* Switch to stable version of .NET framework reference assemblies

* Bump dotnet to get fix to nuget restore for unsigned packages

Related to https://github.com/NuGet/Home/issues/7574

* Update .travis.yml

* Update .travis.yml

* Update .travis.yml

Bump to latest stable

.travis.yml

@@ -5,6 +5,6 @@ script:
   - msbuild /t:Restore $TRAVIS_BUILD_DIR/Terminal.sln
   - cd $TRAVIS_BUILD_DIR/
   - msbuild /p:Configuration=Release Terminal.sln
-mono: "5.20.1"
-dotnet: "2.1.4"
+mono: "6.8.0"
+dotnet: "3.1.201"
 os: "osx"

Terminal.Gui/Terminal.Gui.csproj

@@ -59,10 +59,7 @@
   <ItemGroup>
     <PackageReference Include="NStack.Core" Version="0.11.0" />
     <PackageReference Include="System.ValueTuple" Version="4.4.0" />
-    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0-preview.2">
-      <PrivateAssets>all</PrivateAssets>
-      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
-    </PackageReference>
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" />
   </ItemGroup>
   <ItemGroup>
   </ItemGroup>