error: javac: invalid flag: --sunapi

It seems that `sunapi` Xlint flag is being removed in JDK 9 and later. Thus result in the build error:

    [javac] javac: invalid flag: -Xlint:all,-sunapi,-serial

https://travis-ci.org/javapathfinder/jpf-core/builds/375542872#L2716

References:
https://stackoverflow.com/questions/49134837/javac-invalid-flag-xlint-sunapi-jdk-9