add workaround for dependency problem

.gitlab-ci.yml

@@ -9,7 +9,8 @@ build:
   image: openjdk:8
   stage: build
   script:
-    - ./gradlew --console=plain --no-daemon assemble jar
+    # see discussion in the answer of https://stackoverflow.com/questions/44911155/gradle-how-to-include-runtimeonly-dependencies-in-javaexec-classpath
+    - ./gradlew --console=plain --no-daemon assemble relast-preprocessor:jar jar
   artifacts:
     paths:
       - build/libs/*.jar