Update nokogiri gem (~> 1.8.0) (#26503).

Patch by Go MAEDA.

git-svn-id: https://svn.redmine.org/redmine/trunk@16885 e93f8b46-1217-0410-a6f0-8f06a7374b81

Gemfile

@@ -13,7 +13,7 @@ gem "roadie-rails", "~> 1.2.1"
 gem "roadie", "~> 3.2.1"
 gem "mimemagic"
 
-gem "nokogiri", "~> 1.7.2"
+gem "nokogiri", "~> 1.8.0"
 gem "i18n", "~> 0.7.0"
 
 # Request at least rails-html-sanitizer 1.0.3 because of security advisories