Test class enumerator

Since all three container implementations have the same code for scanning for test classes, I put it in a common helper class in the main assembly. 
Is there a particular range of allowedVersions that we should be constraining LightInject to? (This restriction automatically gets applied the NuGet package when it's created). For example, we constrain xunit on the below line to >= 1.9 and < 2.0.