Test coverage report enabled.

Hi Tan,

The patch should enable the test coverage report. I have tested with my own account, see the result:
1. https://travis-ci.org/hailiang/golang-rest-server
2. https://coveralls.io/r/hailiang/golang-rest-server

Coveralls might not be able to support multiple Go versions as travis-ci, so it is much easier to support only the latest stable version of Go (currently 1.2.2, but 1.3 will be released very soon).

Best regards,
Hailiang