travisci: include secrets so travis can push to gh-pages
travisci: include secrets so travis can push to gh-pages

This was done based on instructions found on the following pages:
* https://evansosenko.com/posts/automatic-publishing-github-pages-travis-ci/
* http://docs.travis-ci.com/user/encryption-keys/
* https://github.com/icorderi/kinetic-rust/blob/master/.travis.yml

rust-embedded/rust-sysfs-gpio,nastevens/rust-sysfs-gpio,jwilm/rust-sysfs-gpio,marjakm/rust-sysfs-gpio