Replace GITHUB_ORGANIZATION with HUBOT_GITHUB_ORG

Howdy,

As there exists quite a few Hubot scripts that uses `HUBOT_GITHUB_ORG` it seems slightly more standard to use that one instead of than defining a new one.

Examples...
* https://github.com/github/hubot-scripts/blob/91100148d09fcc10f70baf5da395c0e443ff81a1/src/scripts/github-pulls.coffee
* https://www.google.se/webhp?q=HUBOT_GITHUB_ORG#q=%22HUBOT_GITHUB_ORG%22