Bump nokogiri to 1.6.8

Number of important security and bug fixes. See:
https://github.com/sparklemotion/nokogiri/blob/master/CHANGELOG.md#168--2016-06-06

CHANGELOG

@@ -16,6 +16,7 @@ v 8.9.0 (unreleased)
   - Redesign navigation for project pages
   - Fix groups API to list only user's accessible projects
   - Redesign account and email confirmation emails
+  - Bump nokogiri to 1.6.8
   - Use gitlab-shell v3.0.0
   - Use Knapsack to evenly distribute tests across multiple nodes
   - Add `sha` parameter to MR merge API, to ensure only reviewed changes are merged

Gemfile.lock

@@ -390,7 +390,7 @@ GEM
     method_source (0.8.2)
     mime-types (2.99.1)
     mimemagic (0.3.0)
-    mini_portile2 (2.0.0)
+    mini_portile2 (2.1.0)
     minitest (5.7.0)
     mousetrap-rails (1.4.6)
     multi_json (1.11.2)
@@ -401,8 +401,9 @@ GEM
     net-ldap (0.12.1)
     net-ssh (3.0.1)
     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)
     oauth (0.4.7)
     oauth2 (1.0.0)
       faraday (>= 0.8, < 0.10)
@@ -474,6 +475,7 @@ GEM
     parser (2.3.1.0)
       ast (~> 2.2)
     pg (0.18.4)
+    pkg-config (1.1.7)
     poltergeist (1.9.0)
       capybara (~> 2.1)
       cliver (~> 0.3.1)