Upgrade to Java 11 #55

I think this seems reasonable as it is an LTS release. Maybe our policy should be to always use the most recent LTS?

Useful APIs:

https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/ClassLoader.html#getPlatformClassLoader() (to replace getSystemClassLoader() in the bundler/deobfuscator)
https://docs.oracle.com/en/java/javase/11/docs/api/jdk.jartool/jdk/security/jarsigner/JarSigner.html (to replace shelling out - we could also replace shelling out to keytool with KeyStore at the same time)
Optional.isEmpty (to replace !isPresent)

Done!