Bump version to 2.3.3

Bump version to 2.3.2

Merge pull request kishikawakatsumi#170 from kishikawakatsumi/release…

…-231

Bump version to 2.3.1

Bump version to 2.3.0

Bump version to 2.2.1

Merge pull request kishikawakatsumi#147 from kishikawakatsumi/release…

…-2.2.0

Bump version to v2.2.0

Hotfix build error on watchOS

Merge pull request kishikawakatsumi#122 from kishikawakatsumi/release…

…-201

Bump version to 2.0.1

Merge pull request kishikawakatsumi#77 from kishikawakatsumi/swift-2.0

Swift 2.0

Merge pull request kishikawakatsumi#97 from kishikawakatsumi/travis_x…

…code7

Use Xcode 7 when testing on Travis CI