Update nokogiri dependency
Update nokogiri dependency

According to GitHub, the old one had security vulnerabilities:

- CVE-2017-9050 Critical severity
- CVE-2017-18258 Moderate severity
- CVE-2017-5029 Low severity
- CVE-2015-5312 High severity
- CVE-2016-4658 High severity
- CVE-2015-8806 Moderate severity
- CVE-2015-7499 Moderate severity

Not super concerned here as nokogiri is a dev dependency only so there
is no user-facing impact here, but I want to stop GitHub yammering at
me.

yrro/command-t,wincent/command-t,yrro/command-t,wincent/command-t,wincent/command-t