Update Gradle example deeplearning4j-examples to support Kotlin DSL with the latest Gradle v6.3

I could provide you with a corresponding pull request and wanted to check if this is actually wanted before I waste my time...

I could refactor `build.gradle` to `build.gradle.kts` that supports the statically typed Kotlin DSL and runs without deprecation warnings with the latest version of Gradle.

Also the idiomatic way shipping Gradle projects is actually committing the Gradle binary into the repository. This might not be needed for Maven which remains API stable for years. However what runs with `gradlew build` on your computer will much more likely not run on my or even on your own just 6 months later. Therefore it is highly advisable to ship the Gradle binary to avoid frustration for people "who just want to evaluate..." :)