handle linting a project in pwd

When calling "django-migration-linter ."
be able to handle it (especially with cache)

django_migration_linter/cache.py

@@ -22,7 +22,7 @@ from .utils import split_path
 class Cache(dict):
     def __init__(self, django_folder, cache_path):
         self.django_folder = django_folder
-        project_name = split_path(django_folder)[-2]
+        project_name = split_path(django_folder)[-1]
         self.filename = os.path.join(cache_path, "{0}.pickle".format(project_name))
 
         if not os.path.exists(os.path.dirname(self.filename)):