Migrate to Java 8 Standard Environment

[shaeberling]

Java 8 is fully supported in the AppEngine Standard Environment, hence retrostore should switch:
https://cloudplatform.googleblog.com/2017/09/Java-8-on-App-Engine-Standard-environment-is-now-generally-available.html

After this we can start cleaning up some of the code to make use of Java 8 features.