CI: use node 9 and one job for build and tests

.gitlab-ci.yml

 stages:
-  - build
-  - test
+  - build_and_tests
 
 .nvm_env: &nvm_env
   tags:
@@ -10,16 +9,12 @@ stages:
     - export NVM_DIR="$HOME/.nvm"
     - . "$NVM_DIR/nvm.sh"
     - export PATH="$HOME/.cargo/bin:$PATH"
+    - nvm install 9.4.0
+    - nvm use 9.4.0
   
-build:
+build_and_tests:
   <<: *nvm_env
-  stage: build
-  script:
-    - npm install
-
-test:
-  <<: *nvm_env
-  stage: test
+  stage: build_and_tests
   script:
     - npm install
     - npm test