I've just updated a dependancy and it is now providing a multi release jar. This results in the following error when building:

Can't process class [META-INF/versions/9/com/teamdev/jxbrowser/chromium/internal/JavaVersion.class] (Unsupported class version number [53.0] (maximum 52.0, Java 1.8)))

Proguard doesn't yet support Java 9 but does have a workaround for this, which is to exclude the versions folder for library jars, eg:

--libraryjars somelib.jar(!META-INF/versions/**)

I've added support for this by blindly adding this exclusion for all library jars.

Does this sounds like an acceptable solution? If so I'll put a pull request together.