Remove npm update; add release-2.3 to branches

This command *should* do nothing immediately following an `npm install`, but causes failures due to `tslint` having a dependency on `typescript`.

Also add release-2.3 to our covered branches

Author: RyanCavanaugh

.travis.yml

@@ -18,12 +18,12 @@ branches:
   - master
   - release-2.1
   - release-2.2
+  - release-2.3
 
 install:
   - npm uninstall typescript
   - npm uninstall tslint
   - npm install
-  - npm update
 
 cache:
   directories: