awsGH-72: Configure 'maven-compiler-plugin' to use Java 8

Author: stepio

aws-lambda-java-events/pom.xml

@@ -29,6 +29,12 @@
     </developer>
   </developers>
 
+  <properties>
+    <java.version>1.8</java.version>
+    <maven.compiler.source>${java.version}</maven.compiler.source>
+    <maven.compiler.target>${java.version}</maven.compiler.target>
+  </properties>
+
   <distributionManagement>
     <repository>
       <id>sonatype-nexus-staging</id>