Seems that Proguard doesn't support Java 8 actually. You have to use JDK version 7.
EDIT-1
I have recompiled Proguard to support JDK 8 classes, doesn't support Java 8 new features but if your code doesn't use Java 8 new syntax(e.g. Lambda Expression) should works without problems. Tested with some android projects.
To use replace the file proguard.jar in
android-sdkoolsproguard
with the content of THIS ARCHIVE
EDIT-2
Recently has been released the official Proguard 5 that should address the issue. You can download this from HERE
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