[MINOR] Update version of ffi.

The other updates were made when building the docs, more exactly when
trying to fix "bundle install", since things wouldn't build with the
existing dependencies.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #108 from vanzin/ffi-update.

docs/Gemfile.lock

@@ -13,12 +13,13 @@ GEM
       execjs
     coffee-script-source (1.12.2)
     colorator (1.1.0)
+    concurrent-ruby (1.0.5)
     ethon (0.10.1)
       ffi (>= 1.3.0)
     execjs (2.7.0)
     faraday (0.12.1)
       multipart-post (>= 1.2, < 3)
-    ffi (1.9.18)
+    ffi (1.9.25)
     forwardable-extended (2.6.0)
     gemoji (3.0.0)
     github-pages (142)
@@ -69,10 +70,11 @@ GEM
       octokit (~> 4.0)
       public_suffix (~> 2.0)
       typhoeus (~> 0.7)
-    html-pipeline (2.6.0)
+    html-pipeline (2.8.4)
       activesupport (>= 2)
-      nokogiri (>= 1.5.4)
-    i18n (0.8.6)
+      nokogiri (>= 1.4)
+    i18n (0.9.5)
+      concurrent-ruby (~> 1.0)
     jekyll (3.4.5)
       addressable (~> 2.4)
       colorator (~> 1.0)
@@ -164,10 +166,10 @@ GEM
     mini_portile2 (2.3.0)
     minima (2.1.1)
       jekyll (~> 3.3)
-    minitest (5.10.2)
+    minitest (5.11.3)
     multipart-post (2.0.0)
     net-dns (0.8.0)
-    nokogiri (1.8.2)
+    nokogiri (1.8.4)
       mini_portile2 (~> 2.3.0)
     octokit (4.7.0)
       sawyer (~> 0.8.0, >= 0.5.3)
@@ -193,7 +195,7 @@ GEM
     thread_safe (0.3.6)
     typhoeus (0.8.0)
       ethon (>= 0.8.0)
-    tzinfo (1.2.3)
+    tzinfo (1.2.5)
       thread_safe (~> 0.1)
     unicode-display_width (1.3.0)
 
@@ -206,4 +208,4 @@ DEPENDENCIES
   rouge
 
 BUNDLED WITH
-   1.15.1
+   1.16.4