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.

managed_vms/helloworld/pom.xml

@@ -49,8 +49,8 @@
         <version>3.3</version>
         <artifactId>maven-compiler-plugin</artifactId>
         <configuration>
-          <source>1.8</source>
-          <target>1.8</target>
+          <source>1.7</source>
+          <target>1.7</target>
         </configuration>
       </plugin>
       <plugin>