Fix build after package upgrade.

test/Piranha.Analyzers.Test/Helpers/DiagnosticVerifier.Helper.cs

@@ -33,7 +33,7 @@ namespace TestHelper
         private static readonly MetadataReference SystemCoreReference = MetadataReference.CreateFromFile(typeof(Enumerable).Assembly.Location);
         private static readonly MetadataReference CSharpSymbolsReference = MetadataReference.CreateFromFile(typeof(CSharpCompilation).Assembly.Location);
         private static readonly MetadataReference CodeAnalysisReference = MetadataReference.CreateFromFile(typeof(Compilation).Assembly.Location);
-        private static readonly MetadataReference PiranhaReference = MetadataReference.CreateFromFile(typeof(Piranha.Models.Content).Assembly.Location);
+        private static readonly MetadataReference PiranhaReference = MetadataReference.CreateFromFile(typeof(Piranha.Models.ContentBase).Assembly.Location);
 
         internal static string DefaultFilePathPrefix = "Test";
         internal static string CSharpDefaultFileExt = "cs";