chore: travis-ci config and gradle 3.4.1

[tompluess]

• added travis-ci config
  (see build result here: https://travis-ci.org/tompluess/example-ratpack-gradle-java-app/builds/211511788 )
• upgrade to gradle 3.4.1
• com.github.jengelman.gradle.plugins:shadow not required anymore / does not work with gradle 3.4.1

[tompluess]

Hi @rhart
What do you think about upgrading this example gradle ratpack project to the newest gradle version?
Additionally i propose to have a travis-ci configuration included.
Regards, Tom.

[rhart]

Thanks @tompluess . Ideally we change the latest branch and merge to master. I'll take care of this, thanks for the contribution.