Kotlin Spring-Boot tutorial

This was made following this tutorial.

However, it refused to run, so we modified it. Not it should run.

Fixes:

• the default Gradle version should be updated: set the gradle/wrapper/gradle-wrapper.properties property distributionUrl to https\://services.gradle.org/distributions/gradle-4.7-rc-1-all.zip (or newer)
• the Kotlin version in build.gradle should be downgraded to 1.1.3, otherwise Gradle will search in /src/main/java for the main class