[Security] Bump mini_magick from 4.9.3 to 4.9.4

Bumps mini_magick from 4.9.3 to 4.9.4. **This update includes security fixes.**

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

Gemfile.lock

@@ -175,7 +175,7 @@ GEM
       mime-types-data (~> 3.2015)
     mime-types-data (3.2019.0331)
     mimemagic (0.3.3)
-    mini_magick (4.9.3)
+    mini_magick (4.9.4)
     mini_mime (1.0.2)
     mini_portile2 (2.4.0)
     minitest (5.11.3)