Fully reverted to stop error with package manager
'dependencies' swiftLanguageVersions: [3], ^~~~~~~~~~~~~~~~~~~~~~~~~~ dependencies: [] Can't parse Package.swift manifest file because it contains invalid format. Fix Package.swift file format and try again.
If you're not using a Swift 4 capable compiler you should probably be using SwiftString v1.0.x instead, though @iamjono will know more.
This package is used by Perfect-Session and the change / partial revert @iamjono made, caused this to break.
swiftLanguageVersions: [3], <- Is unnecessary.
Ah, I didn't realize Perfect-Session used it. Still, I'm running Swift 3.1.1 with no problems, and I'm using both Perfect-Session and I added SwiftString 2 to my Package.swift as well. What version of Swift are you trying to use?