add 'set -e' to bin/hubot

While testing https://github.com/github/hubot/pull/732, I wanted to make sure the most common hubot failures were caught. One of those is incorrect changes to package.json, and it wasn't getting caught.

This adds `set -e` to bin/hubot, which means if any commands exit with non-0, the whole script will fail.

This was added to github/hubot in https://github.com/github/hubot/pull/732