ci: update .travis.yml since add react and react-dom as devDependencies

Author: aquariuslt

.travis.yml

@@ -1,14 +1,13 @@
 language: node_js
 node_js:
-    - "8"
     - "10"
+    - "12"
 env:
     - REACT=16.x
 script:
     - npm test
     - npm run coverage
 before_install:
     - npm install
-    - npm install react react-dom
 after_script:
     npm install coveralls && nyc report --reporter=text-lcov | coveralls