Bump version to MINOR release 2.13.0

Added:
- Add conv.canvas.state feature
- Add express-serve-static-core dependency
- Add test script for CI, update `test:ci`

Change-Id: I979e6250db4c05639154365a887cf80eb37173a3

package.json

@@ -3,7 +3,7 @@
   "description": "Actions on Google Client Library for Node.js",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",
-  "version": "2.12.0",
+  "version": "2.13.0",
   "license": "Apache-2.0",
   "author": "Google Inc.",
   "engines": {