Update .gitlab-ci.yml to run CI for merge requests

.gitlab-ci.yml

@@ -2,6 +2,10 @@ image: danteev/texlive
 
 compile-thesis:
   stage: build
+  rules:
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+    - if: $CI_COMMIT_TAG
+    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
   script:
     - latexmk -pdf thesis
   artifacts: