Specify the build command

Travis defaults to just running `rake` but on newer ruby versions on
Travis, rake v. 12 is already installed which results in
```
Gem::LoadError: You have already activated rake 12.0.0, but your Gemfile requires rake 10.5.0. Prepending `bundle exec` to your command may solve this.
```

.travis.yml

@@ -6,6 +6,8 @@ before_script:
   - bundle exec rails g impressionist -f
   - bundle exec rake db:create db:migrate RAILS_ENV=test
   - cd ..
+script:
+  - bundle exec rake
 rvm:
   - 1.9.3
   - 2.0.0