Push to the correct "gh-pages" remote

The current behavior is to push to "origin" regardless of the actual
remote that is configured for the "gh-pages" branch. This change will
use the configured remote name, defaulting to "origin" if all else
fails.

Signed-off-by: Michael Steinert <mike.steinert@gmail.com>
Merge pull request #60 from msteinert/preserve-remote

Push to the correct "gh-pages" remote