The project is setup with Maven 3.3.2 and JDK 1.6. It seems the Jenkins agent jar was built using JDK 1.8.

Jenkins should be able to build projects in older JDKs even though Jenkins itself is using JDK 1.8

Marked as "blocker" because I cannot find a workaround at this time, please adjust accordingly if not correct.

Possibly related to: 

Parsing POMs
Modules changed, recalculating dependency graph
Established TCP socket on 40120
Copied maven32-agent.jar
Copied maven32-interceptor.jar
Copied maven3-interceptor-commons.jar
[tomitribe-tomee] $ /usr/lib/jvm/java-6-oracle/bin/java -Xmx12g -XX:MaxPermSize=512m -Dhttps.protocols=TLSv1.1,TLSv1.2 -cp /home/ubuntu/maven32-agent.jar:/home/ubuntu/tools/hudson.tasks.Maven_MavenInstallation/maven-3.2.5/boot/plexus-classworlds-2.5.2.jar:/home/ubuntu/tools/hudson.tasks.Maven_MavenInstallation/maven-3.2.5/conf/logging jenkins.maven3.agent.Maven32Main /home/ubuntu/tools/hudson.tasks.Maven_MavenInstallation/maven-3.2.5 /tmp/slave.jar /home/ubuntu/maven32-interceptor.jar /home/ubuntu/maven3-interceptor-commons.jar 40120
Exception in thread "main" java.lang.UnsupportedClassVersionError: jenkins/maven3/agent/Maven32Main : Unsupported major.minor version 51.0
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
Could not find the main class: jenkins.maven3.agent.Maven32Main. Program will exit.
ERROR: Failed to launch Maven. Exit code = 1