Filter out signatures from jars merged into uber-jar.

[balhoff]

This was causing:

[2020-05-19T08:10:48.429Z] Error: A JNI error has occurred, please check your installation and try again
[2020-05-19T08:10:48.429Z] Exception in thread "main" java.lang.SecurityException: Invalid signature file digest for Manifest main attributes
[2020-05-19T08:10:48.429Z]      at sun.security.util.SignatureFileVerifier.processImpl(SignatureFileVerifier.java:330)
[2020-05-19T08:10:48.429Z]      at sun.security.util.SignatureFileVerifier.process(SignatureFileVerifier.java:263)
[2020-05-19T08:10:48.429Z]      at java.util.jar.JarVerifier.processEntry(JarVerifier.java:318)Using ../metadata/datasets for datasets
[2020-05-19T08:10:48.429Z] 
[2020-05-19T08:10:48.429Z]      at java.util.jar.JarVerifier.update(JarVerifier.java:230)
[2020-05-19T08:10:48.429Z]      at java.util.jar.JarFile.initializeVerifier(JarFile.java:383)
[2020-05-19T08:10:48.429Z]      at java.util.jar.JarFile.ensureInitialization(JarFile.java:612)
[2020-05-19T08:10:48.429Z]      at java.util.jar.JavaUtilJarAccessImpl.ensureInitialization(JavaUtilJarAccessImpl.java:69)
[2020-05-19T08:10:48.429Z]      at sun.misc.URLClassPath$JarLoader$2.getManifest(URLClassPath.java:991)
[2020-05-19T08:10:48.429Z]      at java.net.URLClassLoader.defineClass(URLClassLoader.java:451)
[2020-05-19T08:10:48.429Z]      at java.net.URLClassLoader.access$100(URLClassLoader.java:74)
[2020-05-19T08:10:48.429Z]      at java.net.URLClassLoader$1.run(URLClassLoader.java:369)
[2020-05-19T08:10:48.429Z]      at java.net.URLClassLoader$1.run(URLClassLoader.java:363)
[2020-05-19T08:10:48.429Z]      at java.security.AccessController.doPrivileged(Native Method)
[2020-05-19T08:10:48.429Z]      at java.net.URLClassLoader.findClass(URLClassLoader.java:362)
[2020-05-19T08:10:48.429Z]      at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
[2020-05-19T08:10:48.429Z]      at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
[2020-05-19T08:10:48.429Z]      at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
[2020-05-19T08:10:48.429Z]      at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:495)

[balhoff]

Fixes #300.