Switching to preview build

appveyor.yml

@@ -1,4 +1,4 @@
-image: Visual Studio 2019
+image: Visual Studio 2019 Preview
 configuration: Release
 platform: Any CPU
 

tests/Unity.Microsoft.DependencyInjection.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netcoreapp2.2</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <IsPackable>false</IsPackable>
     <RootNamespace>Unity.Microsoft.DependencyInjection.Tests</RootNamespace>
   </PropertyGroup>
@@ -19,12 +19,4 @@
     <ProjectReference Include="..\src\Unity.Microsoft.DependencyInjection.csproj" />
   </ItemGroup>
 
-  <ItemGroup Condition="'$(TargetFramework)' != '' AND '$(TargetFramework)' == 'netcoreapp1.1' ">
-    <PackageReference Include="Microsoft.Extensions.DependencyInjection.Specification.Tests" Version="[1.1.1]" />
-  </ItemGroup>
-
-  <ItemGroup Condition="'$(TargetFramework)' != '' AND '$(TargetFramework)' == 'netcoreapp2.0' ">
-    <PackageReference Include="Microsoft.Extensions.DependencyInjection.Specification.Tests" Version="2.2.0" />
-  </ItemGroup>
-
 </Project>