add sudo: false to .travis.yml to use container-based infrastructure

As per

http://docs.travis-ci.com/user/migrating-from-legacy/?utm_source=legacy-notice&utm_medium=banner&utm_campaign=legacy-upgrade

Author: radar

.travis.yml

@@ -1,6 +1,7 @@
 language: ruby
 before_script: bundle exec rake forem:dummy_app --trace
 script: bundle exec rspec spec
+sudo: false
 env:
   - DB=postgres
   - DB=mysql