fix compatibility with Android plugin 0.10

Fixes this error with the plugin 0.10.x

 ```
* Exception is:
	[... removed for brevity]
Caused by: org.codehaus.groovy.runtime.InvokerInvocationException: groovy.lang.MissingMethodException: No signature of method: com.android.build.gradle.AppPlugin.getSdkDirectory() is applicable for argument types: () values: []
	at org.gradle.listener.ClosureBackedMethodInvocationDispatch.dispatch(ClosureBackedMethodInvocationDispatch.java:40)
	at org.gradle.listener.ClosureBackedMethodInvocationDispatch.dispatch(ClosureBackedMethodInvocationDispatch.java:25)
	at org.gradle.listener.BroadcastDispatch.dispatch(BroadcastDispatch.java:79)
	at org.gradle.listener.BroadcastDispatch.dispatch(BroadcastDispatch.java:31)
	at org.gradle.messaging.dispatch.ProxyDispatchAdapter$DispatchingInvocationHandler.invoke(ProxyDispatchAdapter.java:93)
	at com.sun.proxy.$Proxy14.afterEvaluate(Unknown Source)
	at org.gradle.configuration.project.LifecycleProjectEvaluator.notifyAfterEvaluate(LifecycleProjectEvaluator.java:67)
	... 49 more
Caused by: groovy.lang.MissingMethodException: No signature of method: com.android.build.gradle.AppPlugin.getSdkDirectory() is applicable for argument types: () values: []
	at de.felixschulze.gradle.SpoonPlugin.createTask(SpoonPlugin.groovy:78)
	at de.felixschulze.gradle.SpoonPlugin$_applyTasks_closure2.doCall(SpoonPlugin.groovy:56)
	at org.gradle.api.internal.ClosureBackedAction.execute(ClosureBackedAction.java:58)
	at org.gradle.api.internal.DefaultDomainObjectCollection.all(DefaultDomainObjectCollection.java:110)
	at org.gradle.api.internal.DefaultDomainObjectCollection.all(DefaultDomainObjectCollection.java:115)
	at org.gradle.api.DomainObjectCollection$all$0.call(Unknown Source)
	at de.felixschulze.gradle.SpoonPlugin.applyTasks(SpoonPlugin.groovy:55)
	at de.felixschulze.gradle.SpoonPlugin$_applyExtensions_closure1.doCall(SpoonPlugin.groovy:46)
	... 56 more
```