Use language:node_js instead of manually using nvm and npm

Author: kzys

.travis.yml

@@ -58,10 +58,9 @@ matrix:
         - cargo clippy --all-targets --all-features --all
         - cargo test
     - name: Frontend
-      rust: stable
-      install:
-        - nvm install 10
-        - npm ci
+      language: node_js
+      node_js: 10
+      install: skip
       before_script: skip
       script:
         - npm run lint:js