unpin mysql2 and update

I can't see a reason for us to pin a specific version of this gem.

Closes #532

Gemfile

@@ -2,7 +2,7 @@ source "https://rubygems.org"
 
 gem "rails", "~> 5.2.0"
 
-gem "mysql2", "~> 0.4.10"
+gem "mysql2"
 
 # uncomment to use PostgreSQL
 # gem "pg"

Gemfile.lock

@@ -110,7 +110,7 @@ GEM
     mini_mime (1.0.0)
     mini_portile2 (2.3.0)
     minitest (5.11.3)
-    mysql2 (0.4.10)
+    mysql2 (0.5.2)
     nio4r (2.3.1)
     nokogiri (1.8.4)
       mini_portile2 (~> 2.3.0)
@@ -237,7 +237,7 @@ DEPENDENCIES
   json
   listen
   mail
-  mysql2 (~> 0.4.10)
+  mysql2
   nokogiri (>= 1.7.2)
   oauth
   rails (~> 5.2.0)