Removed incorrect warning from application.js templates and replaced it with stub command description

Full discussion here: https://github.com/rails/sprockets-rails/issues/56

Blank line warning in application.js is wrong. Here what is says:

    // WARNING: THE FIRST BLANK LINE MARKS THE END OF WHAT'S TO BE PROCESSED, ANY BLANK LINE SHOULD
    // GO AFTER THE REQUIRES BELOW.

In fact this is not true, the following compiles just fine:

     //= require jquery
     //= require jquery_ujs
     //= require_tree .

     //= require turbolinks

In fact `stub` directive should be used: https://github.com/sstephenson/sprockets#the-stub-directive