Correct contributing part of README according to travis config

As I can see we _always_ need to use dummy app for testing.

We should define ```BUNDLE_GEMFILE``` env variable or set correct default gemfile path in dummy application:
replace
```ruby
ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../../Gemfile', __FILE__)
```
by
```ruby 
ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../../../../Gemfile', __FILE__)
```
everywhere in dummy app