Trying to get Rails to work with Ruby 1.9 again. Kept getting the error

ArgumentError (invalid byte sequence in US-ASCII):

Sticking this at the top of environment.rb seems to have fixed it:

if RUBY_VERSION =~ /1.9/
Encoding.default_external = Encoding::UTF_8
Encoding.default_internal = Encoding::UTF_8
end

Found the solution here.