I enabled proguard and got:
Warning:can\'t write resource [META-INF/LICENSE.txt] (Duplicate zip entry [commons-io-2.4.jar:META-INF/LICENSE.txt])
Warning:can\'t w
Adding option -ignorewarnings
in the proguard config file worked for me. It still gives warning for 'META-INF/LICENSE.txt' but build does not fail. But use this option only if you are sure about what are its effects. Follow http://proguard.sourceforge.net/manual/usage.html#ignorewarnings for more info.