Update package versions for System.Composition

Author: dsplaisted

src/System.Composition.Convention/tests/project.json

@@ -4,11 +4,7 @@
     "System.Reflection.Emit.Lightweight": "4.0.0-beta-*",
     "System.Runtime": "4.0.20-beta-*",
     "System.Runtime.Serialization.Primitives": "4.0.10-beta-*",
-    "xunit": "2.0.0-beta5-build2785",
-    "xunit.abstractions.netcore": "1.0.0-prerelease",
-    "xunit.assert": "2.0.0-beta5-build2785",
-    "xunit.core.netcore": "1.0.1-prerelease",
-    "xunit.runner.visualstudio": "0.99.9-build1021",
+    "xunit": "2.1.0-beta3-*",
     "xunit.netcore.extensions": "1.0.0-prerelease-*"
   },
   "frameworks": {