[ci] Update codecov configuration

* Explicitly disable code coverage reports for travis CI.
  We use Circle CI now.
* Require the CI to pass before uploading coverage reports.
  This is needed now since we overwrite the default notify settings.

codecov.yml

@@ -1,5 +1,10 @@
 fixes:
   - "::src/api/"
+codecov:
+  ci:
+    - !travis-ci.org
+  notify:
+    require_ci_to_pass: yes
 coverage:
   status:
     project: