Set permission for fallback view to NO_PERMISSION_REQUIRED

If a default permission is set for all views, the fallback view will
fail with a 403 Unauthorized. Invalid Method, Accept, or Content-Type
predicate mismatches will not produce the correct status and error
messages.

Fixes: https://github.com/mozilla-services/cornice/issues/245
Merge pull request #247 from ltvolks/bugfix/245-fallback-permission

Set permission for fallback view to NO_PERMISSION_REQUIRED