Use print_function from __future__

[sedrubal]

To get compatible to python3

See #214

[micahflee]

Sorry about ignoring this for so long. It looks like this alone doesn't bring full python3 compatibility. When I merge it (after resolving the conflicts), the imports are still using the python2 syntax:

$ python3 torbrowser-launcher
Traceback (most recent call last):
  File "torbrowser-launcher", line 29, in <module>
    import torbrowser_launcher
  File "/home/user/code/torbrowser-launcher/torbrowser_launcher/__init__.py", line 33, in <module>
    from common import Common, SHARE
ModuleNotFoundError: No module named 'common'

For now, I'm going to merge #244 to explicitly use python2, but I'd welcome a PR that migrates the whole project to python3, completely dropping python2 compatibility.

[micahflee]

Hah, you already fixed that in #251 I just realized, oops