enforce java 7 on Endpoints sample (GoogleCloudPlatform#448)

* enforce java 7 on Endpoints sample
Currently, mvn appengine:update fails because it compiles Java 8
bytecode. This forces the app code to Java 7.
* set property instead of including plugin

Author: tangiel

appengine/endpoints-frameworks-v2/backend/pom.xml

@@ -35,6 +35,8 @@
         <endpoints.management.version>1.0.0-beta.11</endpoints.management.version>
 
         <endpoints.project.id>YOUR_PROJECT_ID</endpoints.project.id>
+        <maven.compiler.target>1.7</maven.compiler.target>
+        <maven.compiler.source>1.7</maven.compiler.source>
     </properties>
 
     <dependencies>