Upgrade nextflow version.

.gitlab-ci.yml

 before_script:
   - module add  python/3.6.1-2-anaconda
   - pip install --user pytest-pythonpath pytest-cov
-  - module load nextflow/0.27.6
+  - module load  nextflow/0.31.0
   - ln -s /project/shared/bicf_workflow_ref/workflow_testdata/chipseq/*fastq.gz test_data/
 
 stages: