Microsoft.CodeContracts.targets reverts to .NET v3.5 reference assemblies for 4.5.2

[jbcutting]

There's no When block for .NET 4.5.2, so the CodeContractsReferenceAssemblyLibPath falls back to v3.5 contracts for projects that target 4.5.2. Consider adding a block for 4.5.2, a StartsWith('4.5') check can work for any 4.5 release, defaulting to 4.5 rather than 3.5, or using Substring() on the TargetFrameworkVersion to avoid hard-coded versions.