zulip_bots: Use package_data instead of MANIFEST.in.

This commit removes generate_manifest.py and package data files
are now included using the package_data kwarg to setup().

This is because, in certain situations, MANIFEST.in is a bit
finicky. For instance installing a package using:

python setup.py install

doesn't include files specified in MANIFEST.in, while using

pip install ./zulip_bots

does. package_data doesn't pose this problem, ergo it's better
for us.

zulip_bots/README.md

@@ -21,6 +21,5 @@ zulip_bots  # This directory
 │   ├───test_lib.py  # Backbone for bot unit tests.
 │   ├───test_run.py  # Unit tests for run.py
 │   └───terminal.py  # Used to test bots in the command line.
-├───generate_manifest.py  # Helper-script for packaging.
 └───setup.py  # Script for packaging.
 ```