Add GitHub access token to list of output redactions

[chrismytton]

Problem

We added a CleanedOutput class in #64 which removes any mentions of the API key in the build output. We should add the GitHub access token to the list of redactions, so that it doesn't leak out. This will be even more valuable once #67 is done, since more of the build output will be sent to Rollbar if there's an error, so we don't want to leak credentials.

Proposed solution

Add github.access_token to the list of redactions when creating the CleanedOutput instance in app.rb.