Require Tornado < 5

Tornado 5 requires Python >= 2.7.9 because of improvements made to the
`ssl` module in that version. Mopidy probably works with Tornado 5, but
to keep tests running with Python 2.7.6 on Travis CI we don't support
Tornado 5 for now.

setup.py

@@ -27,7 +27,7 @@ setup(
         'Pykka >= 1.1',
         'requests >= 2.0',
         'setuptools',
-        'tornado >= 3.2',
+        'tornado >= 3.2, < 5',  # Tornado 5 requires Python >= 2.7.9
     ],
     extras_require={'http': []},
     entry_points={