Fix Sprockets complaining about the lack of a manifest

There is an issue with Sprockets 4 not accepting a custom path for
the assets manifest, which doesn't play well with in-memory dummy
apps such as the one we use in this gem.

A fix was provided for sprockets-rails[1] but it was not accepted
yet.

[1]: rails/sprockets-rails#446

Gemfile

@@ -9,5 +9,6 @@ gem 'solidus_core', github: 'solidusio/solidus', branch: branch
 gemspec
 
 gem 'solidus_extension_dev_tools', github: 'solidusio-contrib/solidus_extension_dev_tools'
+gem 'sprockets', '~> 3'
 gem 'sprockets-rails'
 gem 'sqlite3'