Only ignore MANIFEST file in root directory

On systems with case-insensitive file systems (like Mac OS X) the previous line
also ignores all types' manifest files.
This commit changes the .gitignore to only ignore the MANIFEST file in the root
directory generated for the PyPI release.

.gitignore

@@ -34,7 +34,7 @@ cdist/inventory/
 # Python: cache, distutils, distribution in general
 __pycache__/
 *.pyc
-MANIFEST
+/MANIFEST
 dist/
 cdist/version.py
 cdist.egg-info/