Update Nokogiri to 1.8.2

There is one security issue found in the lixml2 (2.9.5) vendored in nokogiri: https://github.com/sparklemotion/nokogiri/issues/1714

• omnibus-gitlab is not vulnerable since it already ships lixml2@2.9.6: https://github.com/sparklemotion/nokogiri/issues/1714#issuecomment-361113310
• source installation however are still vulnerable since we're currently using nokogiri@1.8.1

These issues are fixed in nokogiri@1.8.2.

@kathyw Does this need to be fixed in a security release or should we just update the gem in a normal release workflow?

• Ruby security mailing list: https://groups.google.com/forum/#!msg/ruby-security-ann/p3CHpWZDqMA/QDbJRY8LAgAJ
• Nokogiri issue: https://github.com/sparklemotion/nokogiri/issues/1714
• CVE-2017-15412: https://people.canonical.com/%7Eubuntu-security/cve/2017/CVE-2017-15412.html

/cc @stanhu @rspeicher