Merge branch 'dev-gitlab-ci-master-only' into 'master'

modify GitLab CI pipelines to build on master commits

See merge request !47

.gitlab-ci.yml

@@ -15,7 +15,7 @@ build_darshan:
       when: always
     - if: '$CI_PIPELINE_SOURCE == "schedules"'
       when: always
-    - if: '$CI_MERGE_REQUEST_TARGET_BRANCH_NAME == "master"'
+    - if: '$CI_COMMIT_REF_NAME == "master"'
       when: always
 
   stage: build