[Swift4] Update Alamofire dependency

[moveitonover]

The current Alamofire dependency for Swift is 4.7.0 and the current stable version is 4.8.1 with 5.0 in BETA. The major issue with the current dependency is the use of deprecated synax in the Request extension located in ResponseSerilization.swift along with use of 'let' properties that are implicitly 'final' and should use 'public' instead of 'open', in a multitude of components, causing compiler warnings (see attached screenshot).

Suggested fix
The Alamofire dependency should be updated to the latest stable version, optionally monitoring updates to avoid similar problems in the future.

[wing328]

@thecivillain thanks for reporting the issue. May I know if you've time to contribute a PR to update the Alamofire dependency to the latest version? I can show you some good starting points.