* Replace gh-release with curl

This fix tries to address the issue raised in 1895 where
gh-release is deprecated.

As GitHub provides API that could be accessed easily,
and gh-release itself is just a wrapper to the API,
this fix replaces gh-release with the direct interaction
with GitHub API, so that it could be maintained by coredns
team.

This fix fixes 1895.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Added `-o /dev/null`
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>