Upgrade mysql2 gem now that we're on rails 5

Gemfile

@@ -12,9 +12,8 @@ gem "dotenv-rails", "< 2.8.0"
 # Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
 gem "rails", "5.0.7.2"
 
+gem "mysql2"
 gem "sqlite3"
-# The very latest mysql2 gem versions only work with rails 5
-gem "mysql2", "~> 0.4.10"
 
 gem "bootstrap-sass"
 gem "bootstrap-select-rails"
@@ -59,6 +58,9 @@ gem "cancan"
 gem "d3-rails", "~> 3.5"
 # Locking elasticsearch to 7.5.0 as upgrading it seems to break things working
 # with the ancient version of elasticsearch (1.x) that we're running currently
+# This lock is also stopping us from upgrading faraday which is giving us
+# deprecation warnings.
+# TODO: Upgrade elasticsearch as soon as we can
 gem "elasticsearch", "7.5.0"
 gem "haml-coderay"
 gem "honeybadger"

Gemfile.lock

@@ -324,7 +324,7 @@ GEM
     multipart-post (2.2.3)
     mustermann (2.0.2)
       ruby2_keywords (~> 0.0.1)
-    mysql2 (0.4.10)
+    mysql2 (0.5.4)
     nenv (0.3.0)
     net-scp (3.0.0)
       net-ssh (>= 2.6.5, < 7.0.0)
@@ -638,7 +638,7 @@ DEPENDENCIES
   memory_profiler
   meta-tags
   multiblock
-  mysql2 (~> 0.4.10)
+  mysql2
   nio4r (~> 2.4.0)
   nokogiri
   octokit (~> 4.0)