Set encoding in Rakefile to fix "\xC3" on US-ASCII error.

rake aborted!
"\xC3" on US-ASCII
/Users/lu/.rvm/gems/ruby-2.0.0-p0/gems/json-1.8.1/lib/json/common.rb:155:in `encode'
/Users/lu/.rvm/gems/ruby-2.0.0-p0/gems/json-1.8.1/lib/json/common.rb:155:in `initialize'
/Users/lu/.rvm/gems/ruby-2.0.0-p0/gems/json-1.8.1/lib/json/common.rb:155:in `new'
/Users/lu/.rvm/gems/ruby-2.0.0-p0/gems/json-1.8.1/lib/json/common.rb:155:in `parse'
/Users/lu/jquery-ui-rails/Rakefile:38:in `block in map_dependencies'
/Users/lu/jquery-ui-rails/Rakefile:37:in `each'
/Users/lu/jquery-ui-rails/Rakefile:37:in `map_dependencies'
/Users/lu/jquery-ui-rails/Rakefile:54:in `<top (required)>'
/Users/lu/.rvm/gems/ruby-2.0.0-p0/bin/ruby_noexec_wrapper:14:in `eval'
/Users/lu/.rvm/gems/ruby-2.0.0-p0/bin/ruby_noexec_wrapper:14:in `<main>'
Modify Rakefile to introduce dependency declarations for images.
Add automatically generated assets with dependency declarations.