switch to using github api v4

[jmcampanini]

we should look into switching to using the graphql version of the API for GHE. this would give us pushed timestamps, making invalidate_on_push a lot simpler.

a sample query for this would look like:

{
  repository(owner: "gradle", name: "gradle") {
    pullRequest(number: 5813) {
      comments(first: 100) {
        pageInfo {
          endCursor
          hasNextPage
        }
        nodes {
          body
          createdAt
        }
      }
      reviews(first: 100) {
        nodes {
          createdAt
          author {
            login
          }
          state
        }
      }
      commits(first: 100) {
        nodes {
          commit {
            pushedDate
            author {
              user {
                login
              }
            }
            committer {
              user {
                login
              }
            }
          }
        }
      }
    }
  }
}

As a note, we still need to use the v3 API for changed files, since these are not exposed via GraphQL at this time.

[bluekeyes]

Instead of paging the three elements individually, #37 converts the Timeline API calls to use GraphQL. I suppose we could also go back to the pre-invalidate_on_push implementation, where each method on pull.Context made it's own API call. But given that all evaluations need all three of these lists, getting everything from the timeline is convenient (even if it includes events we ignore).