Fix #1025 - Stop using Thread._stop incorrectly when stopping ThreadedArbiter.

[templed]

Also restrict tornado to v<5 (this is done because in v5 the initializer arguments have changed for some of the classes).

I tried using http://issue2pr.herokuapp.com/ to facilitate this PR but it didn't work for me.

It's arguable to either use the dunder syntax or just rearrange the inheritance order of ThreadedArbiter. I went with the former because I prefer it, so I can change what I've done if it's preferable to do the latter :)

[k4nar]

Your PR looks good, but our CI seems to still be broken :( . I have to find some time to fix it.

[coveralls]

Coverage decreased (-0.03%) to 62.94% when pulling a6ae42d on commodityvectors:ThreadedArbiter-stop into 4863345 on circus-tent:master.

[templed]

So I spent a bit of time getting the tests to pass. There is still a weird InvocationError with nosetests that I don't understand for the Python 3.5 suite, but my experience is with pytest so I'm somewhat in the dark.

[templed]

OK I did some more tweaks to the CI setup and on my repo the py34/py35 tests worked (https://travis-ci.org/commodityvectors/circus/builds/339206732) but have failed here for a reason I thought was resolved by the pyenv trick.
I'll leave it for tonight but I'm quite eager to get this merged as that would avoid me having to deal with a local copy of circus.

[dvf]

It looks like Circus is broken for anyone using pipenv by this, as transitive dependencies are upgraded doing a pipenv lock 😢

[k4nar]

The main commit was cherry-picked in 29b0bf3. Thank you for your work and sorry for the awful delay!