feat(github-status): allow sending multiple tokens to be used

This is a port of the changes in travis-ci/travis-core#332 relevant to
travis-tasks after the dependency on travis-core was removed.

It should be completely backwards compatible.
Merge pull request #10 from travis-ci/hh-accept-multiple-tokens

feat(github-status): allow sending multiple tokens to be used
refactor(github-status): extract tokens var to separate method