feat: added Codecov upload of coverage output

.travis.yml

@@ -8,5 +8,8 @@ env:
   - GO111MODULE=on
 
 script:
-  - go test ./... -v -coverprofile=cov.out
-  - go tool cover -func=cov.out
+  - go test ./... -v -coverprofile=coverage.txt -covermode=atomic
+  - go tool cover -func=coverage.txt
+
+after_success:
+  - bash <(curl -s https://codecov.io/bash)