Adding MANIFEST.in to include static files like version.txt

setup.py uses find_packages to look for the files to include when generating the .egg file, and version.txt seems to not be included.

You can see that looking into the supervisor.egg-info/SOURCES.txt metadata, the version.txt file is missing.

In supervisor/options.py file line 56 that file is read, and when the code pass through that lines, supervisord fails.

Adding that file makes setup.py includes version.txt into the .egg file.