Change Hello World compiler to Java 7.
Change Hello World compiler to Java 7.

This sample is linked to from https://cloud.google.com/java/ as "App
Engine flexible environment for Java 7"
https://cloud.google.com/appengine/docs/flexible/java/hello-world. We
should make sure it really is compatible with Java 7 by making sure we
use that compiler instead of Java 8.

ErinJoan/mostly_empty_directory,tswast/java-docs-samples,jetty-project/java-docs-samples,ErinJoan/mostly_empty_directory,jetty-project/java-docs-samples,GoogleCloudPlatform/java-docs-samples,tswast/java-docs-samples,ErinJoan/mostly_empty_directory,jetty-project/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,ErinJoan/mostly_empty_directory,jetty-project/java-docs-samples,GoogleCloudPlatform/java-docs-samples,tswast/java-docs-samples,GoogleCloudPlatform/java-docs-samples,tswast/java-docs-samples,GoogleCloudPlatform/java-docs-samples,jetty-project/java-docs-samples