Bump mini_magick from 3.8.1 to 4.9.4

Bumps [mini_magick](https://github.com/minimagick/minimagick) from 3.8.1 to 4.9.4.
- [Release notes](https://github.com/minimagick/minimagick/releases)
- [Commits](minimagick/minimagick@v3.8.1...v4.9.4)

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile

@@ -62,7 +62,7 @@ gem 'vpim', '~> 13.11.11'
 # for logos + attachments
 gem 'carrierwave', '~> 0.10.0'
 gem 'rmagick', '~> 2.16.0'
-gem 'mini_magick', '~> 3.8.1'
+gem 'mini_magick', '~> 4.9.4'
 
 # global configurations
 gem 'configatron', '~> 2.13.0'

Gemfile.lock

@@ -282,8 +282,7 @@ GEM
       thin (~> 1.5.0)
     method_source (0.8.2)
     mime-types (1.25.1)
-    mini_magick (3.8.1)
-      subexec (~> 0.2.1)
+    mini_magick (4.9.4)
     mini_portile2 (2.0.0)
     minitar (0.5.4)
     minitest (5.8.4)
@@ -480,7 +479,6 @@ GEM
       activesupport (>= 3.0)
       sprockets (>= 2.8, < 4.0)
     sqlite3 (1.3.9)
-    subexec (0.2.3)
     systemu (2.6.4)
     temple (0.6.9)
     terminal-table (1.4.5)
@@ -597,7 +595,7 @@ DEPENDENCIES
   lograge
   logstash-event
   mailcatcher
-  mini_magick (~> 3.8.1)
+  mini_magick (~> 4.9.4)
   momentjs-rails (>= 2.8.1)
   mysql2 (~> 0.3.0)
   net-ldap