Downgrade nokogiri to latest stable version which includes security fixes

https://github.com/sparklemotion/nokogiri/blob/v1.6.6.4/CHANGELOG.rdoc

Gemfile

@@ -85,7 +85,7 @@ gem 'liquid', '~> 3.0.3'
 gem 'mini_magick'
 gem 'mysql2', '~> 0.3.16'
 gem 'multi_xml'
-gem 'nokogiri', '1.6.7.rc4'
+gem 'nokogiri', '1.6.6.4'
 gem 'omniauth'
 gem 'rails', '4.2.4'
 gem 'rufus-scheduler', '~> 3.0.8', require: false

Gemfile.lock

@@ -298,7 +298,7 @@ GEM
     method_source (0.8.2)
     mime-types (2.6.1)
     mini_magick (4.2.3)
-    mini_portile2 (2.0.0.rc2)
+    mini_portile (0.6.2)
     minitest (5.8.1)
     mqtt (0.3.1)
     multi_json (1.11.2)
@@ -311,8 +311,8 @@ GEM
       net-ssh (>= 2.6.5)
     net-ssh (2.9.2)
     netrc (0.10.3)
-    nokogiri (1.6.7.rc4)
-      mini_portile2 (~> 2.0.0.rc2)
+    nokogiri (1.6.6.4)
+      mini_portile (~> 0.6.0)
     oauth (0.4.7)
     oauth2 (0.9.4)
       faraday (>= 0.8, < 0.10)
@@ -587,7 +587,7 @@ DEPENDENCIES
   multi_xml
   mysql2 (~> 0.3.16)
   net-ftp-list (~> 3.2.8)
-  nokogiri (= 1.6.7.rc4)
+  nokogiri (= 1.6.6.4)
   omniauth
   omniauth-37signals
   omniauth-dropbox