Update MANIFEST

Needed to update the requirements lines in MANIFEST. I changed it
so that we shouldn't have to update it again even if we add more
to it.

MANIFEST.in

@@ -1,4 +1,4 @@
 include README.rst CHANGELOG LICENSE CONTRIBUTORS
-include requirements.txt requirements-extra.txt
+include requirements*.txt
 include docs/Makefile docs/conf.py
 recursive-include docs *.rst *.py