This indeed looks like a bug in 1.15, because 1.13 and 1.14 recognize the expired certificate:
$ wget www.cloudflarechallenge.com --2014-08-19 13:41:45-- http://www.cloudflarechallenge.com/ Resolving www.cloudflarechallenge.com (www.cloudflarechallenge.com)... 107.170.194.215 Connecting to www.cloudflarechallenge.com (www.cloudflarechallenge.com)|107.170.194.215|:80... connected. HTTP request sent, awaiting response... 301 Moved Permanently Location: https://www.cloudflarechallenge.com/ [following] --2014-08-19 13:41:46-- https://www.cloudflarechallenge.com/ Connecting to www.cloudflarechallenge.com (www.cloudflarechallenge.com)|107.170.194.215|:443... connected. ERROR: The certificate of 'www.cloudflarechallenge.com' is not trusted. The certificate has expired
