Remove codecov.io

While it was a nice idea, some tests like the backend integration tests
required credentials which were not available to third-party PRs. This
lead to a lot of uncovered code which confused people. So let's disable
codecov.io for now.

.travis.yml

@@ -56,6 +56,3 @@ install:
 
 script:
   - go run run_integration_tests.go
-
-after_success:
-  - test -r all.cov && bash <(curl -s https://codecov.io/bash) -f all.cov