Specify Swift 4.2 in podspecs

[notjosh]

Hiya 👋!

I bumped my project to Swift 5, and started getting some classic "this isn't Swift 5" errors:

Add () to forward @autoclosure parameter

Looking into the podspec, PiwikTracker.podspec specifies Swift 4.1 (via #267, #260), but MatomoTracker.podspec doesn't mention anything at all. Meanwhile .swift-version specifies Swift 4.2.

This PR brings them all into line as Swift 4.2 consistently.

And just in case anyone else is having issues, here's the workaround in your Podfile until this is published:

post_install do |installer|
  swifts = {
    'MatomoTracker' => '4.2'
    # ...specify any other Pods with Swift 4.x requirements here if necessary
  }

  installer.pods_project.targets.each do |target|
    target.build_configurations.each do |config|
      config.build_settings['SWIFT_VERSION'] = swifts[target.name] if swifts.has_key?(target.name)
    end
  end
end

[brototyp-bot]

	1 Warning
:warning:	Are there any changes that should be explained in the README.md?

Generated by :no_entry_sign: Danger

[brototyp]

Hey @notjosh, thanks a lot! Looks really good.

[notjosh]

Do you think you'd be able to bump a 6.0.1 release with support for this some time soon?

[brototyp]

Hey @notjosh, absolutely. Thanks for asking. I just released v6.0.1 including this.