This fix adds the codecov.io process with travis-ci integration,
so that code coverage could be processed and displayed with codecov.io.

Since go cannot use test profile flag with multiple packages,
we have to iterate through packages with `coverage.sh`.

Some consolidation could be done by combining them into one `make test`.

Will do some consolidation in a follow up PR.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>