You need to use JDK 1.7.0 rather than JDK 1.8.0.
To make sure it, you need to delete JDK 1.8.0 on your computer.
If you use Mac, you need to delete:
/Library/Java/JavaVirtualMachines/jdk.jdk
/Library/PreferencePanes/JavaControlPanel.prefPane
/Library/Internet Plug-Ins/JavaAppletPlugin.plugin
Then, you need to reinstall JDK 1.7.0, and you will succeed to generate the .jar file.