Fix Gemfile.lock issue.

Author: matz

Gemfile.lock

@@ -196,7 +196,7 @@ GEM
     tzinfo (1.2.2)
       thread_safe (~> 0.1)
     unicode-display_width (1.1.3)
-    yard (0.9.8)
+    yard (~> 0.9.11)
     yard-coderay (0.1.0)
       coderay
       yard