Revert "Maintenance: Downgrade the mysql2 gem to the previous v0.5.4 to see if this fixes segfaults in CI."

This reverts commit 500f73559a5619cbb7f32e0309aaf32645897886.

Gemfile

@@ -19,7 +19,7 @@ gem 'unicorn', group: :unicorn
 
 # core - supported ORMs
 gem 'activerecord-nulldb-adapter', group: :nulldb
-gem 'mysql2', '0.5.4', group: :mysql # 0.5.5 produced segfaults in CI.
+gem 'mysql2', group: :mysql
 gem 'pg', '~> 1.2.0', group: :postgres
 
 # core - asynchrous task execution

Gemfile.lock

@@ -287,7 +287,7 @@ GEM
     multi_json (1.15.0)
     multi_xml (0.6.0)
     multipart-post (2.3.0)
-    mysql2 (0.5.4)
+    mysql2 (0.5.5)
     naught (1.1.0)
     nestful (1.1.4)
     net-ftp (0.2.0)
@@ -669,7 +669,7 @@ DEPENDENCIES
   messagebird-rest
   mime-types
   minitest-profile
-  mysql2 (= 0.5.4)
+  mysql2
   net-ftp
   net-http
   net-imap