Xcode 10.2: Swift 5.0 migration check -> SWIFT_VERSION = 4.2
Xcode 10.2: Swift 5.0 migration check -> SWIFT_VERSION = 4.2

opening the project in Xcode 10.2 prior to this patch
results in a diagnostic warning that guides the developer
through a Swift 5.0 migration.

doing so results in no code changes.

therefore, this patch does 2 things:
- marks the Swift-migration attempt complete with the
  "LastSwiftMigration = 1020;" change so the warning
  is silenced
- manually sets the setting "SWIFT_VERSION = 4.2" so
  that those still using Xcode 10.1 can continue to
  build and deploy PhoneNumberKit.framework with the
  compiler built into the tools (while still allowing
  those using the Swift 5.0 compiler in Xcode 10.2
  in 4.2 compatibility mode to continue to be able
  take advantage of deploying with the Swift 5.0
  runtime.

marmelroy/PhoneNumberKit,Jean-Daniel/PhoneNumberKit,Jean-Daniel/PhoneNumberKit,Jean-Daniel/PhoneNumberKit,marmelroy/PhoneNumberKit,marmelroy/PhoneNumberKit,marmelroy/PhoneNumberKit