[Security] Bump nokogiri from 1.8.2 to 1.8.4

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.8.2 to 1.8.4. **This update includes security fixes.**
- [Release notes](https://github.com/sparklemotion/nokogiri/releases)
- [Changelog](https://github.com/sparklemotion/nokogiri/blob/master/CHANGELOG.md)
- [Commits](sparklemotion/nokogiri@v1.8.2...v1.8.4)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Author: dependabot-support

Gemfile

@@ -39,7 +39,7 @@ gem "i18n", '~> 0.8.1'
 gem "bootstrap-sass", '~> 2.3.2.2' # locked because blacklight 4.7
 gem "bower-rails", "~> 0.11.0"
 gem "responders", "~> 2.4.0"
-gem 'nokogiri', '1.8.2'
+gem 'nokogiri', '1.8.4'
 
 gem 'uglifier', '~> 3.2.0'
 gem 'rspec-mocks', '3.6.0'

Gemfile.lock

@@ -226,7 +226,7 @@ GEM
     net-ssh (4.1.0)
     netrc (0.10.3)
     newrelic_rpm (4.1.0.333)
-    nokogiri (1.8.2)
+    nokogiri (1.8.4)
       mini_portile2 (~> 2.3.0)
     nom-xml (0.5.3)
       activesupport (>= 3.2.18)
@@ -458,7 +458,7 @@ DEPENDENCIES
   logstash-event (= 1.2.02)
   minitest (~> 5.10.2)
   newrelic_rpm (= 4.1.0.333)
-  nokogiri (= 1.8.2)
+  nokogiri (= 1.8.4)
   omniauth (~> 1.7.1)
   omniauth-shibboleth (~> 1.2.1)
   pg (~> 0.18.2)