Update nokogiri for patches to dependencies

* Security issue from ruby advisory:
sparklemotion/nokogiri#1473
Denial of service or RCE from libxml2 and libxslt
Solution: upgrade to >= 1.6.8

Gemfile.lock

@@ -169,7 +169,7 @@ GEM
       mime-types (>= 1.16, < 3)
     method_source (0.8.2)
     mime-types (2.99.1)
-    mini_portile2 (2.0.0)
+    mini_portile2 (2.1.0)
     minitest (5.8.4)
     multi_json (1.11.2)
     multipart-post (2.0.0)
@@ -180,11 +180,13 @@ GEM
       net-ssh (>= 2.6.5)
     net-ssh (2.9.2)
     newrelic_rpm (3.12.1.298)
-    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)
     normalize-rails (3.0.3)
     oj (2.14.3)
     pg (0.18.2)
+    pkg-config (1.1.7)
     pry (0.10.1)
       coderay (~> 1.1.0)
       method_source (~> 0.8.1)