Setting sinatra version

By running ```bundle install``` the latest version of sinatra gets installed with version 1.4.2. However, the ```set``` macro is not recognized in that version, they changed it in 1.4.0.

This was the error I received:

```
NoMethodError - undefined method `set' for #<Sinatra::Application:0x007fa6131afbd8>:
```

The latest 1.3.x is version 1.3.6, I am proposing either pulling in this pull request or fixing the app for the latest version of sinatra.