Fix open source .pyre_configuration file (#104)

Summary:

The open source pyre configuration is broken.
This is causing Github CI errors: `Invalid configuration: Invalid path ujson: does not exist.`

Differential Revision: D49093816

.pyre_configuration

@@ -3,6 +3,7 @@
     "setup.py",
     "stubs"
   ],
+  "site_package_search_strategy": "pep561",
   "search_path": [
     {
       "site-package": "click"
@@ -34,9 +35,6 @@
     {
       "site-package": "pygments"
     },
-    {
-      "site-package": "ujson"
-    },
     {
       "site-package": "xxhash"
     },
@@ -46,9 +44,6 @@
     {
       "site-package": "traitlets"
     },
-    {
-      "site-package": "typing_extensions"
-    },
     {
       "site-package": "pyre_extensions"
     },