Fix up the project.json files for CoreFX build rc2-23922.

- ProjectModel uses AssemblyLoadContext which is only available in netstandard1.5. Moving ProjectModel and all its dependencies up to netstandard1.5.
- Work around our dependencies still being on old CoreFX builds by hoisting System.Runtime.Serialization.Primitives, System.Private.DataContractSerialization, System.Linq.Expressions, and System.Xml.XmlSerializer references to the current version.

TestAssets/TestProjects/ProjectWithTests/project.json

@@ -2,6 +2,8 @@
   "version": "1.0.0-*",
   "dependencies": {
     "NETStandard.Library": "1.5.0-rc2-23922",
+    "System.Linq.Expressions": "4.0.11-rc2-23922",
+    "System.Runtime.Serialization.Primitives": "4.1.1-rc2-23922",
     "xunit": "2.1.0",
     "dotnet-test-xunit": "1.0.0-dev-91790-12"
   },