Drop python2 support

[muupan]

Resolves #463
Resolves #392
Resolves #467

• Travis CI with python 2 is stopped
• future is removed from dependencies
• import from __future__ is removed
• hacks for python 2 are removed (Support Python2/3 JSON dumps of dicts. #392)
• switched from ppa:jonathanf to ppa:cran for installing ffmpeg since the former is no longer available
• remove use of six
• remove fastcache, funcsigs, statistics as dependencies

[muupan]

/test

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[muupan]

/test

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[toslunar]

Could you remove six?

% git grep 'import six'
chainerrl/misc/makedirs.py:import six
chainerrl/replay_buffers/episodic.py:import six.moves.cPickle as pickle
chainerrl/replay_buffers/replay_buffer.py:import six.moves.cPickle as pickle
tests/misc_tests/test_random.py:import six

LGTM except for it.

[muupan]

/test

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[muupan]

Good point! I removed six. By this changechainerrl.misc.makedirs is now completely the same as os.makedirs, so I removed the file.

[muupan]

/test

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[muupan]

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