Fix androidJavadocs

Converting class com.android.build.gradle.internal.api.DefaultAndroidSourceDirectorySet to File using toString() method has been deprecated and is scheduled to be removed in Gradle 2.0. Please use java.io.File, java.lang.String, java.net.URL, or java.net.URI instead.
Merge pull request #38 from grantland/master

Fix androidJavadocs