Maintenance: Update dependency mysql2 to v0.5.6

Gemfile.lock

@@ -344,7 +344,7 @@ GEM
     multi_json (1.15.0)
     multi_xml (0.6.0)
     multipart-post (2.3.0)
-    mysql2 (0.5.5)
+    mysql2 (0.5.6)
     naught (1.1.0)
     nestful (1.1.4)
     net-ftp (0.3.4)