Include distribute_setup.py in the MANIFEST to ship it in sdists.

MANIFEST.in

@@ -1,5 +1,5 @@
-include setup.py README.rst MANIFEST.in LICENSE
+include setup.py README.rst MANIFEST.in LICENSE distribute_setup.py
 recursive-include sentry/templates *
 recursive-include sentry/static *
 recursive-include sentry/plugins/*/templates *
-global-exclude *~
+global-exclude *~