Add Dictionary and List dependency to dnxcore50

Author: BrennanConroy

src/Microsoft.AspNet.WebUtilities/project.json

@@ -16,6 +16,7 @@
         "dnx451": { },
         "dnxcore50": {
             "dependencies": {
+                "System.Collections": "4.0.11-beta-*",
                 "System.Diagnostics.Debug": "4.0.11-beta-*",
                 "System.IO": "4.0.11-beta-*",
                 "System.IO.FileSystem": "4.0.1-beta-*",