Remove Chainer v2 test from Travis

[delta2323]

Since Chainer v4 has been released, Chainer installed by CHAINER_VERSION="chainer in travis has been changed to from v4 to v3. That means Chainer v3 is not tested in the current configuration. This PR adds a test with Chainer v4 to .travis.yml.

I did not remove configuration about Chainer v2 (CHAINER_VERSION="chainer<3) because testing time in Travis is not a bottleneck in the current workflow. But I would be like opinions.

[codecov-io]

Codecov Report

Merging #154 into master will increase coverage by 1.84%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
+ Coverage   77.54%   79.39%   +1.84%     
==========================================
  Files          79       81       +2     
  Lines        3385     3562     +177     
==========================================
+ Hits         2625     2828     +203     
+ Misses        760      734      -26

[corochann]

I think building miniconda takes most of the time. Does it build miniconda 12 times?

[delta2323]

Does it build miniconda 12 times?

Yes, in total. The reason for using miniconda is that conda is a recommended way to install RDKit (if I remember correctly) and we can expect most users install in this way.

[corochann]

I actually feel the time for building miniconda is not so short, and actually test takes some sort of time.
Do you think the overhead is acceptable?
Is there any way to make test faster, e.g., use docker image etc?

[delta2323]

I feel sometimes many jobs are queued (e.g. before releasing a new version), so it would be better to reduce testing time.

Currently we test with v2, 4, and 5, I think at least we should replace v2 with v3, if we do not want to increase the number of configurations in the CI.

[corochann]

I feel if we want to support v2 - latest, we should test v2 (edge of the supporting version).

[delta2323]

An alternative way is to remove tests with Python 3.5.

[delta2323]

I remove tests with Python 3.5 as to keep testing time. It is just a workaround and we need to consider a better way.

[delta2323]

Let me close this PR and resend it.