Update to latest nunit

Author: kevinkuszyk

samples/FluentAssertions.AspNetCore.Mvc.Sample.Tests/FluentAssertions.AspNetCore.Mvc.Sample.Tests.csproj

@@ -21,7 +21,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.0.0-preview-20170106-08" />
-    <PackageReference Include="NUnit" Version="3.4.1" />
+    <PackageReference Include="NUnit" Version="3.8.1" />
     <PackageReference Include="dotnet-test-nunit" Version="3.4.0-beta-1" />
   </ItemGroup>
 

tests/FluentAssertions.AspNetCore.Mvc.Tests/FluentAssertions.AspNetCore.Mvc.Tests.csproj

@@ -24,7 +24,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.0.0-preview-20170106-08" />
-    <PackageReference Include="NUnit" Version="3.4.1" />
+    <PackageReference Include="NUnit" Version="3.8.1" />
     <PackageReference Include="dotnet-test-nunit" Version="3.4.0-beta-1" />
     <PackageReference Include="Microsoft.AspNetCore.Mvc" Version="1.0.3" />
     <PackageReference Include="Moq" Version="4.6.36-alpha" />