Update scala/app snippet to extend App

Earlier versions of Scala let its users extend a trait `Application` to
avoid boilerplate when writing a main program. The `Application` trait
was deprecated in Scala 2.9, and replaced by the `App` trait
(`Application` was subsequently removed in Scala 2.11). This commit
replaces `Application` with `App` in the `app` snippet.
Merge pull request #53 from christian-soderberg/update-scala-app

Update scala/app snippet to extend App