[TASK] Configure Travis for better build performance

* set sudo: false so Travis can use their new container-based infrastructure
* enable bundler caching

Author: oliverklee

.travis.yml

@@ -1,4 +1,6 @@
+sudo: false
 language: ruby
+cache: bundler
 env:
   - USE_MINI_PORTILE=true
   - USE_MINI_PORTILE=false