Force project system test project to use Immutable 1.1.37

This prevents it from deploying 1.1.36 to the bin dir when its built

src/VisualStudio/ProjectSystem/DeployTestDependencies/project.json

@@ -3,7 +3,8 @@
     "xunit": "2.1.0",
     "xunit.runner.console": "2.1.0",
     "xunit.runner.visualstudio": "2.1.0",
-    "Moq": "4.2.1402.2112"
+    "Moq": "4.2.1402.2112",
+    "System.Collections.Immutable": "1.1.37"
   },
   "frameworks": {
     "net46": { }