Add (redundant) using declaration to fix compilation on mono 2.x, that was throwing this error:

Unit/Diagnostics/CustomInteractiveDiagnosticsFixture.cs(104,36): error CS1928: Type `string' does  not contain a
 member `ShouldContain' and the best extension method overload
 `Nancy.Testing.AssertExtensions.ShouldContain(this Nancy.Testing.NodeWrapper, string, System.StringComparison)'
 has some invalid arguments