Fix Ruby 1.8.7 warning about regex syntax.

Noticed this when running against Ruby 1.8.7, there were two repeated
warnings based on improper escaped brackets:

- spec/rspec/rails/matchers/route_to_spec.rb:114:
  > warning: regexp has invalid interval

- spec/rspec/rails/matchers/route_to_spec.rb:114:
  > warning: regexp has `}' without escape