Eliminate reference to System.Runtime.CompilerServices.Unsafe on .NETCOREAPP2.1

src/ImageSharp/ImageSharp.csproj

@@ -40,12 +40,12 @@
     <PackageReference Include="StyleCop.Analyzers" Version="1.1.0-beta007">
       <PrivateAssets>All</PrivateAssets>
     </PackageReference>
-    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.5.0" />
   </ItemGroup>
 
   <ItemGroup Condition=" '$(TargetFramework)' != 'netcoreapp2.1'">
     <PackageReference Include="System.Buffers" Version="4.5.0" />
     <PackageReference Include="System.Memory" Version="4.5.0" />
+    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.5.0" />
   </ItemGroup>
   
   <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard1.1' OR '$(TargetFramework)' == 'netstandard1.3'">

tests/ImageSharp.Tests/ImageSharp.Tests.csproj

@@ -25,6 +25,11 @@
   <ItemGroup>
     <None Include="PixelFormats\PixelOperationsTests.Blender.cs" />
   </ItemGroup>
+
+  <ItemGroup Condition=" '$(TargetFramework)' == 'netcoreapp2.0'">
+    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.5.0" />
+  </ItemGroup>
+  
   <ItemGroup>
     <PackageReference Include="Microsoft.CSharp" Version="4.5.0" />
     <PackageReference Include="System.Drawing.Common" Version="4.5.0" />