This issue is merely to track our progress on fully supporting / taking advantage of Java 9, as per #582 (comment)

• Add new Java version to the JavaLanguageModule
• Support module-info.java syntax.
• Since we support analyzing sources directly from jar-files, we maybe need to improve our code to support JEP 238: Multi-Release JAR Files (although it might just work - but some classes might be duplicated then and confuse typeresolution and metrics...).
• Support language changes introduced on JEP 213: Milling Project Coin (private interface methods, _ is an illegal identifier)
• Have Travis build and run on JDK 9
• Check for other changes on Java Release Page
• Use JEP 236: Parser API for Nashorn when available instead of Rhino (non-critical for JDK 9 release)