Fix regexp warnings on 1.8.7.

./spec/rspec/mocks/argument_matchers_spec.rb:277: warning: regexp has invalid interval
./spec/rspec/mocks/argument_matchers_spec.rb:277: warning: regexp has `}' without escape
./spec/rspec/mocks/argument_matchers_spec.rb:284: warning: regexp has invalid interval
./spec/rspec/mocks/argument_matchers_spec.rb:284: warning: regexp has `}' without escape
Merge pull request #785 from rspec/fix-regexp-warnings-on-1.8.7

Fix regexp warnings on 1.8.7.