Downgrade .gitlab-ci.yml to support old CI

This commit removes the 'cache:key:files' directive because our CI's
gitlab is currently too old to support it.

.gitlab-ci.yml

@@ -18,14 +18,10 @@ stages:
 stack:
   stage: stack
   cache: 
-    key:
-      files:
-        - stack.yaml
+    key: stack.yaml
     paths:
       - .stack-root/
       - .stack-work/
-      - dist-newstyle/
-      - .cabal/
   script:
     - echo "Building the project from '$CI_PROJECT_DIR'"
     - nix-shell --run "stack build --no-terminal --fast --dry-run"
@@ -33,12 +29,8 @@ stack:
 cabal:
   stage: cabal
   cache: 
-    key:
-      files:
-        - cabal.project
+    key: cabal.project
     paths:
-      - .stack-root/
-      - .stack-work/
       - dist-newstyle/
       - .cabal/
     policy: pull-push
@@ -49,12 +41,8 @@ cabal:
 test:
   stage: test
   cache:
-    key:
-      files:
-        - gargantext.cabal
+    key: cabal.project
     paths:
-      - .stack-root/
-      - .stack-work/
       - dist-newstyle/
       - .cabal/
     policy: pull
@@ -64,18 +52,19 @@ test:
 docs:
   stage: docs
   cache:
+    key: stack.yaml
     paths:
       - .stack-root/
       - .stack-work/
-      - dist-newstyle/
-      - .cabal/
-      - target
     policy: pull
   script:
     - nix-shell --run "stack build --no-terminal --haddock --no-haddock-deps --fast --dry-run"
     - cp -R "$(stack path --local-install-root)"/doc ./output
-  rules:
-    - if: '$CI_MERGE_REQUEST_IID' # Run job on Merge Requests
+  # FIXME(adinapoli) Currently Gitlab 11.x doesn't support the 'rules' keyword.
+  # rules:
+  # - if: '$CI_MERGE_REQUEST_IID' # Run job on Merge Requests
+  only:
+    - merge_requests
   artifacts:
     paths:
       - ./output