chore(ci): execute codecov via npx

* drop codecov devDependency

.travis.yml

@@ -11,7 +11,7 @@ jobs:
         firefox: 'latest-esr'
       script:
         - COVERAGE=1 TEST_BROWSERS=Firefox,PhantomJS xvfb-run npm run all
-        - codecov
+        - npx codecov
     - stage: update integration test
       script: './tasks/stages/update-integration-test'
     - stage: update examples

package.json

@@ -41,7 +41,6 @@
   "devDependencies": {
     "chai": "^4.1.2",
     "chai-match": "^1.1.1",
-    "codecov": "^3.1.0",
     "cpx": "^1.5.0",
     "del": "^3.0.0",
     "eslint": "^4.19.0",