_tracking page should only respond to POST

The `_tracking` page when hit with a GET request throws a traceback. It should ideally just accept `POST`.

```
Stacktrace (most recent call last):

  File "raven/middleware.py", line 35, in __call__
    iterable = self.application(environ, start_response)
  File "ckan/config/middleware.py", line 355, in __call__
    k, v = part.split('=')

```