bundle update nokogiri, security fixes

    $ bundle-audit check
    Name: nokogiri
    Version: 1.6.7.2
    Advisory: CVE-2015-8806
    Criticality: Unknown
    URL: sparklemotion/nokogiri#1473
    Title: Denial of service or RCE from libxml2 and libxslt
    Solution: upgrade to >= 1.6.8

    Vulnerabilities found!

Gemfile.lock

@@ -55,21 +55,23 @@ GEM
       multi_json (~> 1.10)
     memoist (0.12.0)
     method_source (0.8.2)
-    mini_portile2 (2.0.0)
+    mini_portile2 (2.1.0)
     minitest (5.4.3)
     multi_json (1.11.0)
     multi_xml (0.5.5)
     multipart-post (2.0.0)
     newrelic_rpm (3.8.1.221)
-    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)
       jwt (~> 1.0)
       multi_json (~> 1.3)
       multi_xml (~> 0.5)
       rack (~> 1.2)
+    pkg-config (1.1.7)
     pry (0.10.3)
       coderay (~> 1.1.0)
       method_source (~> 0.8.1)
@@ -138,4 +140,4 @@ RUBY VERSION
    ruby 2.2.5p319
 
 BUNDLED WITH
-   1.12.3
+   1.12.4