UnsupportedClassVersionError when running build with jdk 1.5

To reproduce :
Declare JDK 5 in jenkins configuration
Create a job : freestyle project
Configure job : with Ant/Ivy-Artifactory Integration

Stacktrace :

[latest] $ /usr/lib/ant/apache-ant-1.7.1/bin/ant -file build.xml livraison -v -lib /var/lib/jenkins/plugins/artifactory/WEB-INF/lib -listener org.jfrog.build.extractor.listener.ArtifactoryBuildListener
Apache Ant version 1.7.1 compiled on June 27 2008
Buildfile: build.xml

BUILD FAILED
Class org.jfrog.build.extractor.listener.ArtifactoryBuildListener could not be loaded because of an invalid dependency.
at org.apache.tools.ant.util.ClasspathUtils.newInstance(ClasspathUtils.java:296)
at org.apache.tools.ant.Main.addBuildListeners(Main.java:799)
at org.apache.tools.ant.Main.runBuild(Main.java:681)
at org.apache.tools.ant.Main.startAnt(Main.java:217)
at org.apache.tools.ant.launch.Launcher.run(Launcher.java:257)
at org.apache.tools.ant.launch.Launcher.main(Launcher.java:104)
Caused by: java.lang.UnsupportedClassVersionError: Bad version number in .class file
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:621)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
at java.net.URLClassLoader.access$100(URLClassLoader.java:56)
at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:242)
at org.apache.tools.ant.util.ClasspathUtils.newInstance(ClasspathUtils.java:266)
... 5 more
— Nested Exception —
java.lang.UnsupportedClassVersionError: Bad version number in .class file
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:621)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
at java.net.URLClassLoader.access$100(URLClassLoader.java:56)
at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:242)
at org.apache.tools.ant.util.ClasspathUtils.newInstance(ClasspathUtils.java:266)
at org.apache.tools.ant.Main.addBuildListeners(Main.java:799)
at org.apache.tools.ant.Main.runBuild(Main.java:681)
at org.apache.tools.ant.Main.startAnt(Main.java:217)
at org.apache.tools.ant.launch.Launcher.run(Launcher.java:257)
at org.apache.tools.ant.launch.Launcher.main(Launcher.java:104)

Note :
build-info-api-2.0.7.jar
build-info-client-2.0.7.jar
build-info-extractor-2.0.7.jar
build-info-vcs-2.0.7.jar
=> seems to be compile with jdk 1.6