Support Ruby 2.2.0 and 2.3.0

Judging from the test suite, the only issue in supporting Ruby 2.2.0+
was with the `mysql2` gem. Updating this to the latest 0.3.x allows
Lobsters to run with newer Rubies.

We can't update to the absolute latest `mysql2` gem without first
updating Rails, which is why I used the pessimistic version constraint
for `mysql2`.

.ruby-version

@@ -1 +1 @@
-1.9.3-p429
+2.3.0

Gemfile.lock

@@ -56,7 +56,7 @@ GEM
     mime-types (2.6.1)
     mini_portile (0.5.3)
     minitest (5.7.0)
-    mysql2 (0.3.16)
+    mysql2 (0.3.20)
     nokogiri (1.6.1)
       mini_portile (~> 0.5.0)
     oauth (0.4.7)
@@ -147,4 +147,4 @@ DEPENDENCIES
   unicorn
 
 BUNDLED WITH
-   1.10.6
+   1.11.2