Support sprockets-rails3

[yasaichi]

I got the following error when I used this gem with sprockets-rails v3.0.0.

/home/travis/build/yasaichi/gakubuchi/lib/gakubuchi/template_engine.rb:26:in `registered?': undefined method `engines' for nil:NilClass (NoMethodError)
    from /home/travis/build/yasaichi/gakubuchi/lib/gakubuchi/template_engine.rb:17:in `register!'
    from /home/travis/build/yasaichi/gakubuchi/lib/gakubuchi/railtie.rb:4:in `block in <class:Railtie>'
    from /home/travis/.rvm/gems/ruby-2.2.0/gems/railties-4.2.5/lib/rails/initializable.rb:30:in `instance_exec'
    from /home/travis/.rvm/gems/ruby-2.2.0/gems/railties-4.2.5/lib/rails/initializable.rb:30:in `run'
    from /home/travis/.rvm/gems/ruby-2.2.0/gems/railties-4.2.5/lib/rails/initializable.rb:55:in `block in run_initializers'

(You can see the full stack trace here.)

As far as I looked over sprockets-rails, it occurred due to the difference of how to define Rails.application.assets.

In v2.x, it is defined as an instance method of Rails::Application class (ref. here).
Therefore, we can access Sprockets::Environment in initializer blocks.

On the other hand, in v3.0.0, Rails.application.assets is defined by attr_accessor (ref. here).
Then, the value is set in the config.after_initialize block (ref. here).
Therefore, we can access Sprockets::Environment only in config.after_initialize blocks.

In addition, from the latest version, Rails.application.assets always returns nil when config.assets.compile is disabled (ref. README.md).

For these reasons, we must add the following changes to support sprokets-rails3.

• Use config.assets.configure instead of initializer.
• Use other methods than ::Rails.application.assets.find_asset to resove digest paths of templates

[yasaichi]

[IMO]
klass.instance_of?(::Class) seems better.

[yasaichi]

[IMO]
foo.html.erb seems better because in most cases the format of a source file is something other than html.

[yasaichi]

[IMO]
Related to this comment, you should implement this method as follows.

def digest_path_in_context(context)
  # do something
end