Adapt Manifest to 0.4.0

The test/ directory ended up taking a lot of space in the
distributed files (tar.gz, wheel). The whole directory
was hence removed from these files. To further save
some space, the whole checks/ directory was also
removed.

Author: aleju

MANIFEST.in

@@ -3,8 +3,6 @@ include setup.cfg
 include LICENSE
 include MANIFEST.in
 include README.md
-include checks/README.md
 include requirements.txt
 recursive-include imgaug *.py *.jpg *.ttf *.png *.json
-recursive-include test *.py
-recursive-include checks *.py
+prune test