Fix library dependancy issue when running with config.ru

I'm runnig gollum head with the sample config.ru
https://github.com/gollum/gollum#rack
But when I create page, I'm getting following error.

```
NoMethodError - undefined method `encodeURIComponent' for #<Precious::App:0x007fb0b51e5d58>
        /Users/foo/gollum/lib/gollum/app.rb:413:in `show_page_or_file'
        /Users/foo/gollum/lib/gollum/app.rb:385:in `block in <class:App>'
        /Users/foo/.rbenv/versions/1.9.3-p392/lib/ruby/gems/1.9.1/gems/sinatra-1.4.2/lib/
se.rb:1415:in `call'
...
```

It is hard to write test. But this error is reproducible with comment out this line. https://github.com/gollum/gollum/blob/master/test/helper.rb#L16