Update nokogiri to 1.6.8.1

Gemfile

@@ -109,7 +109,7 @@ gem 'liquid', '~> 3.0.3'
 gem 'loofah', '~> 2.0'
 gem 'mini_magick'
 gem 'multi_xml'
-gem 'nokogiri', '1.6.8'
+gem 'nokogiri'
 gem 'omniauth', '~> 1.3.1'
 gem 'rails', '~> 5.0.0.1'
 gem 'rufus-scheduler', '~> 3.0.8', require: false

Gemfile.lock

@@ -341,9 +341,8 @@ GEM
     net-ssh (3.0.2)
     netrc (0.10.3)
     nio4r (1.2.1)
-    nokogiri (1.6.8)
+    nokogiri (1.6.8.1)
       mini_portile2 (~> 2.1.0)
-      pkg-config (~> 1.1.7)
     notiffany (0.0.8)
       nenv (~> 0.1)
       shellany (~> 0.0)
@@ -383,7 +382,6 @@ GEM
       omniauth-oauth2 (~> 1.1)
     orm_adapter (0.5.0)
     pg (0.18.3)
-    pkg-config (1.1.7)
     poltergeist (1.8.1)
       capybara (~> 2.1)
       cliver (~> 0.3.1)
@@ -652,7 +650,7 @@ DEPENDENCIES
   multi_xml
   mysql2 (~> 0.3.20)
   net-ftp-list (~> 3.2.8)
-  nokogiri (= 1.6.8)
+  nokogiri
   omniauth (~> 1.3.1)
   omniauth-37signals
   omniauth-dropbox
@@ -705,4 +703,4 @@ RUBY VERSION
    ruby 2.3.1p112
 
 BUNDLED WITH
-   1.13.2
+   1.13.6