String characters count deprecation

[fassko]

When I integrate Starscream in my lib and run pod lib lint I get deprecation warnings:

- WARN  | xcodebuild:  Starscream/Sources/WebSocket.swift:534:35: warning: 'characters' is deprecated: Please use String or Substring directly
- WARN  | xcodebuild:  Starscream/Sources/WebSocket.swift:810:26: warning: 'characters' is deprecated: Please use String or Substring directly
- WARN  | xcodebuild:  Starscream/Sources/WebSocket.swift:811:33: warning: 'characters' is deprecated: Please use String or Substring directly