build: enable codecov partial coverage and use bash uploader (#1396)

.codecov.yml

@@ -0,0 +1,4 @@
+coverage:
+  parsers:
+    javascript:
+      enable_partials: yes

.travis.yml

@@ -17,5 +17,5 @@ script:
   - npm run test-cov
   - npm run bench
 after_script:
-  - npm install codecov
-  - ./node_modules/.bin/codecov
+  # only upload the coverage.json file
+  - bash <(curl -s https://codecov.io/bash) -f coverage/coverage-final.json