Support for Java 11

[roti]

frege-native-gen does not work when run with JDK 11 (and probably with any JDK > 9). It turns out the problem is here: https://github.com/Frege/frege-native-gen/blob/master/src/main/frege/frege/nativegen/Main.fr#L151

com.google.common.reflect.ClassPath.from always returns an empty collection, and it's likely due to the new module system. An upgrade to Guava 27.1-jre fixed the problem.