disable strict trailing-slash

allows `nbviewer.ipython.org/gist-id/` without 404 error.

As requested by @fperez, specifically for listing multi-file gists with the logical url, such as this:

http://nbtest.herokuapp.com/3693491/

Technically, this also allows *any* gist to be viewed with trailing slash. Due to apparent limitations of werkzeug, it doesn't seem possible to simply redirect 'gist-id/' to 'gist-id', since both will be routed to the same handler.