Use previous VS environment.

appveyor.yml

@@ -1,5 +1,5 @@
 version: 1.0.0.{build}
-image: Visual Studio 2017
+image: Previous Visual Studio 2017
 
 # prevent the double build when a branch has an active PR
 skip_branch_with_pr: true

src/ImageSharp/ImageSharp.csproj

@@ -29,7 +29,7 @@
     <DebugType Condition="$(codecov) == ''">portable</DebugType>
     <DebugSymbols>True</DebugSymbols>
     <Features>IOperation</Features>
-    <LangVersion>latest</LangVersion>
+    <LangVersion>7.3</LangVersion>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="..\Shared\*.cs" Exclude="bin\**;obj\**;**\*.xproj;packages\**" />

tests/ImageSharp.Tests/ImageSharp.Tests.csproj

@@ -2,7 +2,7 @@
   <PropertyGroup>
     <TargetFrameworks>net471;netcoreapp2.0;netcoreapp2.1;net47;net462</TargetFrameworks>
     <AllowUnsafeBlocks>True</AllowUnsafeBlocks>
-    <LangVersion>latest</LangVersion>
+    <LangVersion>7.2</LangVersion>
     <DebugType Condition="$(codecov) != ''">full</DebugType>
     <DebugType Condition="$(codecov) == ''">portable</DebugType>
     <DebugSymbols>True</DebugSymbols>
@@ -22,8 +22,11 @@
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <WarningsAsErrors />
   </PropertyGroup>
-    <ItemGroup>
-    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.5.1" />
+  <ItemGroup>
+    <None Include="PixelFormats\PixelOperationsTests.Blender.cs" />
+  </ItemGroup>
+  
+  <ItemGroup>
     <PackageReference Include="Magick.NET-Q16-AnyCPU" Version="7.5.0" />
     <PackageReference Include="Microsoft.CSharp" Version="4.5.0" />
     <PackageReference Include="System.Drawing.Common" Version="4.5.0" />