Prevent exception by no extension logical_path

Hi @lucasmazza

Thank you for very nice library. Stylus is so comfortable for me ;)
  
I use ruby-stylus and newest sprockets (2.10.1) with bower in Rails4.

When I use some packages which are installed via bower, then I got error __"Asset logical path has no extension xxx/xxx"__ against some files like `README` or `LICENSE` at development server process and `assets:precompile` phase.
This error will be raised at initialization of [Sprockets::Asset](https://github.com/sstephenson/sprockets/blob/master/lib/sprockets/asset.rb#L36) class.

My third-party packages are in `vendor/assets/components`. And I use these packages with `require` directive via bower.json support of sprockets in application.css and application.js.

The ruby-stylus seems to add all files (currently without `css|js`) from `config.assets.paths` of rails into `assets_hash`.
This commit ignores these no extension files also. It works for me.

What do you think about it ?. (Do you know any other solutions ?)
\# This seems to be related sprocket's [issue #347](https://github.com/sstephenson/sprockets/issues/347).

This is my [environment and trace](https://gist.github.com/grauwoelfchen/8646077).