Upgrade to Nginx Pagespeed 1.8.31.4

This corrects a vulnerability in the version of OpenSSL used by
ngx_pagespeed - http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2014-0224

We aren't using HTTPS fetching in pagespeed yet, so this is mainly
cautionary
Merge pull request #107 from washingtonstateuniversity/ugrade/pagespeed

[PR] Upgrade to ngx_pagespeed 1.8.31.4-beta