Bump nokogiri to 1.6.8

https://groups.google.com/forum/#!topic/ruby-security-ann/RCHyF5K9Lbc

Gemfile

@@ -134,7 +134,7 @@ gem 'cocaine'
 # also, better than thin since we can control worker concurrency.
 gem 'unicorn'
 
-gem 'nokogiri', '~> 1.6.7'
+gem 'nokogiri', '~> 1.6.8'
 
 gem 'carrierwave', '~> 0.10.0'
 gem 'fog', '~> 1.23.0', require: 'fog/aws/storage'

Gemfile.lock

@@ -315,7 +315,7 @@ GEM
       mime-types (>= 1.16, < 3)
     method_source (0.8.2)
     mime-types (1.25.1)
-    mini_portile2 (2.0.0)
+    mini_portile2 (2.1.0)
     minitest (5.8.0)
     mixlib-shellout (2.1.0)
     multi_json (1.11.3)
@@ -328,8 +328,9 @@ GEM
     net-ssh (2.9.2)
     netrc (0.11.0)
     newrelic_rpm (3.14.1.311)
-    nokogiri (1.6.7.2)
-      mini_portile2 (~> 2.0.0.rc2)
+    nokogiri (1.6.8)
+      mini_portile2 (~> 2.1.0)
+      pkg-config (~> 1.1.7)
     non-stupid-digest-assets (1.0.4)
     oj (2.11.5)
     parallel (1.6.1)
@@ -342,6 +343,7 @@ GEM
       rake (>= 0.8.1)
     pdf-core (0.6.1)
     pg (0.18.3)
+    pkg-config (1.1.7)
     poltergeist (1.7.0)
       capybara (~> 2.1)
       cliver (~> 0.3.1)
@@ -613,7 +615,7 @@ DEPENDENCIES
   mysql2 (~> 0.3.20)
   net-ldap (~> 0.8.0)
   newrelic_rpm
-  nokogiri (~> 1.6.7)
+  nokogiri (~> 1.6.8)
   non-stupid-digest-assets
   oj (~> 2.11.4)
   omniauth!