Bump version to MINOR release 2.6.0

Release Notes:

Features:
* Add ability to respond with an HTTP 401 Status Code
* Add support for other lambda formats
* Add more error handling for report state

Fixes:
* Fix unhandled exception in jwtClient.authorize

Other Changes:
* Upgrade to TypeScript 3
* Add package security audit to CI process
* Change typedoc theme
* Add source code links for Typedoc config
* Selectively install yarn version 1.13.0 for travis
* Upgrade ava and nyc to resolve yarn audit vulnerabilities

Change-Id: I42888e33f8cd94f9c1a21e9beb7285291ab49353

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": "2.5.0",
+  "version": "2.6.0",
   "license": "Apache-2.0",
   "author": "Google Inc.",
   "engines": {