Respect config.assets.prefix when generating assets

[lsimoneau]

I've updated the Rails engine to make use of config.assets.prefix. Previously, if I set the prefix, when running rake assets:precompile, the compiled JS files would be dumped into the default location (public/assets), and then it would fail complaining that it couldn't find the manifest in the correctly-prefixed location:

$ RAILS_ENV=production rake assets:precompile                                                     [16:26:13]
/Users/louis.simoneau/.rubies/ruby-2.2.0/bin/ruby ./bin/rake requirejs:precompile:all RAILS_ENV=production RAILS_GROUPS=assets
rake aborted!
Errno::ENOENT: No such file or directory @ rb_sysopen - /Users/louis.simoneau/rea/app/public/subdir/assets/rjs_manifest.yml
/Users/louis.simoneau/.gem/ruby/2.2.0/gems/requirejs-rails-0.9.8/lib/tasks/requirejs-rails_tasks.rake:179:in `initialize'
/Users/louis.simoneau/.gem/ruby/2.2.0/gems/requirejs-rails-0.9.8/lib/tasks/requirejs-rails_tasks.rake:179:in `open'
/Users/louis.simoneau/.gem/ruby/2.2.0/gems/requirejs-rails-0.9.8/lib/tasks/requirejs-rails_tasks.rake:179:in `open'
/Users/louis.simoneau/.gem/ruby/2.2.0/gems/requirejs-rails-0.9.8/lib/tasks/requirejs-rails_tasks.rake:179:in `block (4 levels) in <top (required)>'
/Users/louis.simoneau/.gem/ruby/2.2.0/gems/requirejs-rails-0.9.8/lib/tasks/requirejs-rails_tasks.rake:148:in `each'
/Users/louis.simoneau/.gem/ruby/2.2.0/gems/requirejs-rails-0.9.8/lib/tasks/requirejs-rails_tasks.rake:148:in `block (3 levels) in <top (required)>'
Tasks: TOP => requirejs:precompile:all => requirejs:precompile:digestify_and_compress
(See full trace by running task with --trace)
rake aborted!
Command failed with status (1): [/Users/louis.simoneau/.rubies/ruby-2.2.0/b...]
/Users/louis.simoneau/.gem/ruby/2.2.0/gems/requirejs-rails-0.9.8/lib/tasks/requirejs-rails_tasks.rake:18:in `ruby_rake_task'
/Users/louis.simoneau/.gem/ruby/2.2.0/gems/requirejs-rails-0.9.8/lib/tasks/requirejs-rails_tasks.rake:85:in `block (3 levels) in <top (required)>'
Tasks: TOP => assets:precompile => requirejs:precompile:external
(See full trace by running task with --trace)

[Sija]

:+1: