Update to Swift 4.2 + Fix .swift-version file deprecation

[Chris-Perkins]

Hi! Thanks for the great library. While using this library in my own project, my project was failing to build due to some cocoapods changes (highlighted below).

This pull request does the following:

1. Remove the .swift-version file and replace it with a s.swift_version in the Podspec file for the following reason:
   • CocoaPods 1.4.0+ doesn't fully respect .swift-version files like previous installations; installed pods will attempt to be built using your project's explicit swift version.
   • In my case, this was Swift 4.2, and the project failed to build since multiple renames have occurred from Swift 4.0 -> 4.2.
2. Update the files to be compatible with Swift 4.2 to keep up with Swift's constantly evolving/deprecating nature

This pull request does not do the following:

• Update the podspec file with a new version number or create any new tags-- you'll have to do this part if this PR is merged