Support GitHub check-runs API (#58)

These GitHub-driven checks are collected and analyzed
the same way as the (old) commit statuses. For simplicity,
check-runs are stored in the same StatusChecks array.

The 'staging_checks' parameter now reflects the total
number of required commit statuses and required GitHub check-runs.

This support is necessary for projects that
require GitHub Actions.

package.json

@@ -4,7 +4,7 @@
   "description": "A bot that merges GitHub pull requests",
   "main": "Main.js",
   "dependencies": {
-    "@octokit/rest": "^14.0.8",
+    "@octokit/rest": "^18.12.0",
     "bunyan": "^1.8.12",
     "github-webhook-handler": "^0.6.0",
     "parse-github-url": "^0.3.2",