Enable error-prone FallThrough test

[regisd]

In v. 1.7.0 the generated code doesn't have FallThrough violations anymore (see #222).

Re-enable the test as an error (currently downgraded as warning)

[regisd]

Actually, the generated code is still not conformant

/home/travis/build/jflex-de/jflex/jflex/target/generated-sources/jflex/jflex/LexScan.java:2550: error: [FallThrough] Execution may fall through from the previous case; add a `// fall through` comment before this line if it was deliberate
            case 799: break;
            ^
    (see http://errorprone.info/bugpattern/FallThrough)

https://travis-ci.org/jflex-de/jflex/jobs/431900782