Add support for Gradle 3.x

[timpatt]

The plugin fails to load into Gradle 3.x with an error:

Unable to load class us.kirchmeier.capsule.task.FatCapsule due to missing dependency org/gradle/api/internal/DynamicObject

According to this post, Gradle 3 introduced a binary incompatibility that has affected some plugins - "the class DynamicObject has been moved to the package org.gradle.internal.metaobject with Gradle 3.0.". The solution suggested on the post is to simply rebuild and republish the plugin built with Gradle 3.x...