Fix Travis deploy script
Fix Travis deploy script

This change updates our `deploy.sh` script to use two Travis
[environment variables][1] when tagging commits:

 - `$TRAVIS_EVENT_TYPE` which is `pull_request` for Pull Request builds
 - `$TRAVIS_BRANCH` which is the branch name

We unfortunately can't rely on `$TRAVIS_BRANCH` alone, as this is
`master` on Pull Request builds.

[1]: https://docs.travis-ci.com/user/environment-variables/

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