Make it possible to run within mountable engine within a dummy app (using jasminerice)

[pschyska]

We have a mountable engine with dummy app located at spec/dummy
Jasminerice serves the specs up correctly @ /jasmine

guard-jasmine fails to rackup the dummy app, because it is running rackup in the engine dir. Therefore, it hangs at

Guard::Jasmine starts webrick test server on port 8888 in development environment.

Using the verbose option, one can clearly see it's looking for engine_dir/config.ru.

This patch adds the ability to provide a rackup_config option to point to the correct file.

guard :jasmine, :server => :webrick, :rackup_config => 'spec/dummy/config.ru' do
  [...]
end

[netzpirat]

Thanks a lot! Released with version 1.9.0.