Add support for Ubuntu 16.04

[lukeyeager]

Several key packages have been upgraded from 14.04 to 16.04.

• gevent 1.0 -> 1.1.0
• gunicorn 17.5 -> 19.4.5

The gevent upgrade is particularly troublesome because it requires upgrading Flask-Socketio. Which isn't so bad in and of itself, but causes problems for packaging because of a naming conflict (see #476 (comment), miguelgrinberg/python-socketio#23). I can get around the packaging issue, but it's not pretty.

Another issue is that some versions of gunicorn and gevent conflict. So while requirements says that you can use gevent>=1.0,<=1.1.0 and/or gunicorn>=17.5,<=19.4.5, it should really read (gevent==1.0 AND gunicorn==17.5) OR (gevent==1.1.0 AND gunicorn==19.4.5). Otherwise you get this weird behavior where the Caffe subprocess goes defunct.

I also found a funny pydot packaging bug along the way (see commit msg).

[lukeyeager]

Rebased after conflict with #800.

[gheinrich]

I tried this PR on Ubuntu 14.04. I did a pip install -r requirements.txt and created classification datasets/models on both the prod and dev server. I didn't see anything wrong.

[lukeyeager]

Just to try to head off any complaints, if you see this error:

Traceback (most recent call last):
  File "./digits-devserver", line 42, in <module>
    from digits.webapp import app, socketio, scheduler
  File "/home/lyeager/digits/digits/webapp.py", line 22, in <module>
    socketio = SocketIO(app, async_mode='gevent')
TypeError: __init__() got an unexpected keyword argument 'async_mode'

Then you need to run this command to get the new requirements:

pip install -r requirements.txt

In general, you need to run that command anytime there is a change made to requirements.txt (documentation).