Only include PRs that were merged

[tmr08c]

Sometimes a pull request will be closed, but not merged. We should filter out closed but not merged pull requests.

Possible Solutions

1. Use the Pull Request endpoint instead (Use GitHub's Pull Request endpoint #8). The response from this endpoint as a merged_at field. Ideally, this would be empty or something when the PR hasn't been merged.
2. Make an additional call to the Pull Request endpoint to check if merge (https://developer.github.com/v3/pulls/#get-if-a-pull-request-has-been-merged) for each PR we have.