Update to Mapbox-iOS-SDK 6.3.0

The 6.0.0 upgrade had no breaking changes:

> This major release does not include any breaking changes to public
> APIs. We are treating this release as a SEMVER major change because
> our installation instructions have changed.
https://github.com/mapbox/mapbox-gl-native-ios/releases/tag/ios-v6.0.0

I did however need to cast the `accessToken` as a `String` to satisfy
the compiler after this upgrade.

Prior to the upgrade I was seeing crashes from this bug, which was fixed
in 6.2: mapbox/mapbox-gl-native-ios#485

So far I don't see any issues at all after the upgrade.

Developers will need to configure their access token in order to fetch
the mapbox sdk for this an future versions, described here:
https://docs.mapbox.com/ios/maps/guides/install/

README.md

@@ -10,7 +10,8 @@ This Flutter plugin allows to show embedded interactive and customizable vector
 
 - Install [Flutter](https://flutter.io/get-started/) and validate its installation with `flutter doctor`
 - Clone the repository with `git clone git@github.com:tobrun/flutter-mapbox-gl.git`
-- Add a Mapbox access token to the example app (see next section)
+- Add a public Mapbox access token to the example app (see next section)
+- Add a secret Mapbox access token for downloading the SDK
 - Connect a mobile device or start an emulator, simulator or chrome
 - Locate the id of a the device with `flutter devices`
 - Run the app with `cd flutter_mapbox/example && flutter packages get && flutter run -d {device_id}`
@@ -25,6 +26,18 @@ The **recommended** way to provide your access token is through the `MapboxMap`
 
 An alternative method to provide access tokens that was required until the v0.7 release is described in [this wiki article](https://github.com/tobrun/flutter-mapbox-gl/wiki/Mapbox-access-tokens).
 
+### SDK Download token
+
+You must also [configure a secret access token having the Download: read
+scope][https://docs.mapbox.com/ios/maps/guides/install/]. If this configuration
+is not present, an error like the following appears during the iOS build.
+
+```
+[!] Error installing Mapbox-iOS-SDK
+curl: (22) The requested URL returned error: 401 Unauthorized
+```
+
+
 ## Avoid Android UnsatisfiedLinkError
 
 Update buildTypes in `android\app\build.gradle`

ios/Classes/MapboxMapController.swift

@@ -28,7 +28,7 @@ class MapboxMapController: NSObject, FlutterPlatformView, MGLMapViewDelegate, Ma
 
     init(withFrame frame: CGRect, viewIdentifier viewId: Int64, arguments args: Any?, registrar: FlutterPluginRegistrar) {
         if let args = args as? [String: Any] {
-            if let token = args["accessToken"] as? NSString{
+            if let token = args["accessToken"] as? String {
                 MGLAccountManager.accessToken = token
             }
         }

ios/mapbox_gl.podspec

@@ -16,7 +16,7 @@ A new Flutter plugin.
   s.public_header_files = 'Classes/**/*.h'
   s.dependency 'Flutter'
   s.dependency 'MapboxAnnotationExtension', '~> 0.0.1-beta.1'
-  s.dependency 'Mapbox-iOS-SDK', '~> 5.6.0'
+  s.dependency 'Mapbox-iOS-SDK', '~> 6.3.0'
   s.swift_version = '4.2'
   s.ios.deployment_target = '9.0'
 end