bump nokogiri 1.7.1 --> 1.7.2
bump nokogiri 1.7.1 --> 1.7.2

A security alert was raised by gemnasium:
Sources: https://github.com/sparklemotion/nokogiri/issues/1634
http://people.canonical.com/~ubuntu-security/cve/2017/CVE-2017-5029.html
http://people.canonical.com/~ubuntu-security/cve/2016/CVE-2016-4738.html

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