Fix 1.8.7 warning: "regexp has invalid interval"

Since the upgrade to sass 3.3.14 I get this warning (with ruby 1.8.7):

    C:/development/lang/ruby187/lib/ruby/gems/1.8/gems/sass-3.3.14/bin/../lib/sass/script/lexer.rb:343: warning: regexp has invalid interval

Apparently  {  needs to be escaped in Ruby 1.8