Check optional dependencies at runtime

[kmaehashi]

Optional dependencies should not be decleared as requirements, as it makes difficult for users to uninstall the optional dependencies.

One of the common pitfall (when user upgraded their CUDA version):

pip install cupy-cuda90
pip install chainer
# -> pip recognizes the dependency from chainer to cupy-cuda90

pip uninstall cupy-cuda90
pip install cupy-cuda92

pip install chainer
# -> pip tries to "fix" broken dependency, causing installation of cupy-cuda90

[kmaehashi]

Jenkins, test this please.

[chainer-ci]

Jenkins CI test (for commit caea4ee, target branch master) failed with status FAILURE.
(For contributors, please wait until the reviewer confirms the details of the error.)

[takagi]

Jenkins, test this please.

[takagi]

Would you check the CI failure on Travis?

[chainer-ci]

Jenkins CI test (for commit 12df749, target branch master) succeeded!

[kmaehashi]

Fixed. (I'm unsure why it does not fail on Py3.x, though...)

[takagi]

LGTM!