Use pip for installing Chainer/CuPy

[niboshi]

Fixes chainer/chainer#7084

[kmaehashi]

Jenkins, test this please.

[chainer-ci]

Jenkins CI test (for commit 0cc0ece, target branch master) failed with status FAILURE.

[toslunar]

Jenkins, test this please.

[chainer-ci]

Jenkins CI test (for commit 0cc0ece, target branch master) succeeded!

[toslunar]

pip install prints less log than python setup.py install. How about pip install -v?

[niboshi]

Should it be done in this PR?
If so, I want to add -v to all the "pip install" instances in this repository.

[toslunar]

To me, pip install -v looks closer to python setup.py than pip install, while I don't have a good reference.

Is there a reason why pip install and python setup.py were mixed? If you are somewhat confident that the answer is no, it's reasonable to use pip install without -v in the PR.

[niboshi]

I guess the reason was to test multiple ways of installation.
I'm pretty sure it was not for the amount of logs.