Updated remaining demo url in README.
Closes #3 (reported by espeed). Fixed the __request() method
returning r.json instead of r.json().

The problem stemmed from a change in the requests library documented in
in commit fe0b0a989fad80a300d2cf940d58370d13cc9e42 where r.json is
updated to r.json(). To prevent issues with updates in dependencies, I
locked the requests library to version 1.2.* in setup.py.