tornado>=3.0.1,<4.2
futures
PyYAML>=3.0
six>=1.4.0

tornado>=4.0
futures
PyYAML>=3.0
six>=1.4.0

Deal with potential memory leak in tornado AsyncHttpClient
Deal with potential memory leak in tornado AsyncHttpClient

As per the tornado docs,
http://www.tornadoweb.org/en/stable/releases/v4.0.0.html#tornado-httpclient
Tornado 4.0+ deals with a memory leak in AsyncHttpClient shutdown.

This is explained in:
https://github.com/tornadoweb/tornado/commit/8953e9beb1e376d9d907c77e61e0e628245de35d

For convenience:

Clients created without force_instance=True were never getting
GC'd because we were using the wrong reference for the async_clients
instance cache.

To guard against future errors, failure to remove an instance
from the cache when we expected to now raises an error instead of
failing silently.
