build: remove deprecated node-github api (#12420)

Switches to the newer version of `node-github` that has been renamed to `@octokit/rest`.

package.json

@@ -53,6 +53,7 @@
     "@angular/upgrade": "6.0.0",
     "@bazel/ibazel": "0.3.1",
     "@google-cloud/storage": "^1.1.1",
+    "@octokit/rest": "^15.9.4",
     "@schematics/angular": "^0.7.1",
     "@types/chalk": "^0.4.31",
     "@types/fs-extra": "^4.0.3",
@@ -75,7 +76,6 @@
     "firebase-tools": "^3.11.0",
     "fs-extra": "^3.0.1",
     "git-semver-tags": "^2.0.0",
-    "github": "^12.0.3",
     "glob": "^7.1.2",
     "google-closure-compiler": "20170409.0.0",
     "gulp": "^3.9.1",