Allow flask-restful to import with very old versions of flask.
Allow flask-restful to import with very old versions of flask.

We're using a version of flask from 2010 (don't ask).  Everything
works fine actually, except flask.views doesn't exist in such an old
version.  Make it so we just don't define Resource in that case.
Resource probably doesn't make much sense with such an old flask
anyway.

Khan/flask-restful