deploy.sh: Use TRAVIS_REPO_SLUG for target repo name
deploy.sh: Use TRAVIS_REPO_SLUG for target repo name

For convenience to build gh-pages on the forked repos, this patch
fixes to use "TRAVIS_REPO_SLUG" environment variable on Travis-CI.

Signed-off-by: IWASE Yusuke <bb80e84cc9f5a8f01385872ee89032beb5d4eba7@gmail.com>

osrg/ryu-book,iwaseyusuke/ryu-book,iwaseyusuke/ryu-book,osrg/ryu-book