Generate test coverage statistics during Travis runs

[minusnine]

We should generate test coverage statistics during Travis runs. This can be accomplished by:

1. Changing travis-ci-test.sh to:
   a. Add --coverprofile=coverage.go to the go test invocation.
   b. Add a call to go tool cover -html coverage.out -o coverage.html.
2. Configuring the artifact to be uploaded at the end of the test.

2 requires use to configure an Amazon S3 bucket. The files should be configured to expire after a couple of days (maybe 30?), since Travis runs for a branch on each push and daily.

[minusnine]

Instead of uploading to a bucket directly, maybe use Codecov:

https://codecov.io/
https://github.com/codecov/example-go