Add support for forks

[squalrus]

Add the sha as the ref rather than building a string representing the branch name

The ref can be a SHA, branch name, or a tag name.

From https://developer.github.com/v3/checks/runs/#list-check-runs-for-a-git-reference

fixes #40

[squalrus]

@aaron-trout Could you try creating a PR into this branch from your fork to test?

[aaron-trout]

@squalrus sure thing! #42

Update: Whoops, forgot to change the target branch on that one, try #43

[squalrus]

Verified the test run in #43 (after setting the branch to fork-support). Need to revert to master to merge.