Support sprockets-rails ~> 3.2

[obliviusm]

Hello. Let me explain myself

Problem: New version of sprockets-rails is not compatible with princely. In new version of sprockets-rails code Rails.application.assets returns assets for development environment but doesn't return assets in production-like environments.
Solution: I've modified AssetSupport#asset_file_path. So it generates the sprockets environment if it finds Rails.application.assets to be nil.
This solution is taken from Compass/compass-rails#263 . They had the same problem.

In this PR I've done next:

• added dummy rails project that is used in test
• added development dependencies: rails, sprockets-rails
• added tests for AssetSupport#asset_file_path that checks production environment. If you run this test with the old version of AssetSupport#asset_file_path you'll see that it fails.
• modified AssetSupport#asset_file_path as described in solution

Thanks

[pftg]

@obliviusm pls fix broken tests

[repinel]

Not sure about the issue/fix, but having a dummy Rails app to test is definitely something good to have.

@modsognir What do you think if I create a new PR just to add the Rails app test for the current code in master? Later, this PR would be simpler to review.

[pftg]

@repinel there is dummy app in this pr

[repinel]

@repinel there is dummy app in this pr

@pftg Yes, I noticed. I was basically suggesting separating this PR in two.

[modsognir]

Sorry I missed this one folks. @repinel yes, if the dummy app was split into a seperate PR that would make this one a lot easier to check over. Not sure why this is failing on 1.9.3 but I think we might be able to remove that as 1.9.3 hasn't been officially supported in 2 years.

[repinel]

@modsognir I can work on it