Merge pull request optuna#898 from sile/bump-to-v1.1.0

Bump up version number to 1.1.0.

Merge pull request optuna#831 from hvy/bump-to-v1.0.0

Bump up version number to 1.0.0

Merge pull request optuna#709 from optuna/bump-to-v0.19.0

Bump up version number to 0.19.0.

Merge pull request optuna#680 from pfnet/bump-to-v0.18.1

Bump up version number to 0.18.1.

Merge pull request optuna#671 from pfnet/bump-to-v0.18.0

Bump up version number to 0.18.0.

Merge pull request optuna#593 from pfnet/bump-to-v0.17.1

Bump up version number to 0.17.1

Merge pull request optuna#588 from pfnet/bump-to-v0.17.0

Bump up version number to 0.17.0

Merge pull request optuna#512 from pfnet/bump-to-v0.16.0

Bump up version number to 0.16.0.

Merge pull request optuna#499 from pfnet/bump-to-v0.15.0

Bump up version number to 0.15.0.

Merge pull request optuna#468 from pfnet/bump-to-v0.14.0

Bump up version number to 0.14.0.