Resolve GradleFx#217: Properly detect if air needs to be installed

Rely on airsdk.xml instead of lib/adt.jar which happens to be present in
some Flex SDK distributions (e.g. http://download.macromedia.com/pub/flex/sdk/flex_sdk_4.6.zip).

While airsdk.xml hasn't always been present on air distributions, it has
been bundled since air 3.2. Unfortunately there's no single file that
has always been present and that is not present in all Flex
distributions.

src/main/groovy/org/gradlefx/configuration/sdk/states/air/CreateAirSdkInstallLocationState.groovy

@@ -24,7 +24,11 @@ import org.gradlefx.configuration.Configurations
 
 class CreateAirSdkInstallLocationState extends AbstractCreateSdkInstallLocationState {
     CreateAirSdkInstallLocationState(Boolean isInstallationRequired) {
-        super(SdkType.AIR, "lib/adt.jar", Configurations.AIRSDK_CONFIGURATION_NAME.configName(), isInstallationRequired)
+        // lib/adt.jar is unfortunately present in some flex distributions
+        // (e.g. flex 4.6), so it's not a good choice.
+        // airsdk.xml has been present since air 3.2
+        // air-sdk-description.xml since air 4.0
+        super(SdkType.AIR, "airsdk.xml", Configurations.AIRSDK_CONFIGURATION_NAME.configName(), isInstallationRequired)
     }
 
     @Override