Upgrade mysql2 gem

I ran into the issues described here when trying to install necessary
gems to contribute:

brianmario/mysql2#1350

Upgrading seems to have resolved the issue.

Author: jherdman

Gemfile.lock

@@ -81,7 +81,7 @@ GEM
     mocha (2.1.0)
       ruby2_keywords (>= 0.0.5)
     mutex_m (0.2.0)
-    mysql2 (0.5.4)
+    mysql2 (0.5.6)
     nio4r (2.7.0)
     nokogiri (1.16.6-arm64-darwin)
       racc (~> 1.4)