Warnings about deprecated String.characters usage

[1ec5]

CocoaPods is issuing the following warnings when using Xcode 9.1:

 -> OSRMTextInstructions (0.5.0)
    - WARN  | xcodebuild:  OSRMTextInstructions/OSRMTextInstructions/OSRMTextInstructions.swift:18:23: warning: 'characters' is deprecated: Please use String or Substring directly
    - WARN  | xcodebuild:  OSRMTextInstructions/OSRMTextInstructions/OSRMTextInstructions.swift:18:67: warning: 'characters' is deprecated: Please use String or Substring directly
    - WARN  | xcodebuild:  OSRMTextInstructions/OSRMTextInstructions/OSRMTextInstructions.swift:100:102: warning: 'characters' is deprecated: Please use String or Substring directly

These issues would be resolved by migrating to Swift 4.

/cc @bsudekum

[bsudekum]

Will be fixed in #50