Merge pull request ishkawa#279 from ishkawa/release/5.1.0

Set version 5.1.0

Merge pull request ishkawa#260 from ishkawa/release/4.0.0

Set version 4.0.0

Merge pull request ishkawa#254 from ishkawa/release/3.2.1

Set version 3.2.1

Merge pull request ishkawa#252 from ishkawa/swift-4

Xcode 9 / Swift 4

Merge branch 'master' into develop/4.0