Fix using MANIFEST.in for wheel too

sdist always read MANIFEST.in, wheel used `package_data` setting.

MANIFEST.in

@@ -3,4 +3,4 @@ include LICENSE
 include DOCS.rst
 include CHANGES.rst
 recursive-include polymorphic/static *.js *.css
-recursive-include polymorphic/templates *
+recursive-include polymorphic/templates *.html

setup.py

@@ -36,11 +36,7 @@ setup(
     maintainer_email='tribaal@gmail.com',
 
     packages=find_packages(),
-    package_data={
-        'polymorphic': [
-            'templates/admin/polymorphic/*.html',
-        ],
-    },
+    include_package_data=True,
 
     test_suite='runtests',