Remove extraneous . from VALID_FILE_EXTENSIONS

Otherwise if you «include_vars: vars.json», you get this error:

    "message": "/path/to/vars.json does not have a valid extension: yaml, yml, .json"

lib/ansible/plugins/action/include_vars.py

@@ -102,7 +102,7 @@ class ActionModule(ActionBase):
     def run(self, tmp=None, task_vars=None):
         """ Load yml files recursively from a directory.
         """
-        self.VALID_FILE_EXTENSIONS = ['yaml', 'yml', '.json']
+        self.VALID_FILE_EXTENSIONS = ['yaml', 'yml', 'json']
         if not task_vars:
             task_vars = dict()