Merge pull request chainer#2314 from pfnet/_v2-update-v2.0.0a1

Update v2.0.0a1

Merge pull request chainer#2269 from pfnet/update-v1.21.0

v1.21.0

Update version

Merge pull request chainer#2107 from pfnet/v1.20.0

Update version

Merge pull request chainer#1996 from pfnet/v1.19.0

Update version

Merge pull request chainer#1885 from pfnet/ver1.18.0

Update version

Merge pull request chainer#1782 from pfnet/v1.17.0

Update version

Merge pull request chainer#1688 from pfnet/v1.16

Update version

Merge pull request chainer#1619 from pfnet/v1.15.0.1

update version to v1.15.0.1

Merge pull request chainer#1607 from pfnet/v1.15.0

update version to v1.15.0