Skip sintra-assetpack prebuild during tests (faster)

Regarding https://github.com/rstacruz/sinatra-assetpack/issues/101 I think removing the prebuild in test env should do it. 

The prebuild is triggered when the app is loaded, and you are doing so in the tests since you are using an instance of your sinatra app :

https://github.com/swanson/stringer/blob/master/spec/spec_helper.rb#L28