Bump mini_magick from 4.8.0 to 4.9.4

Bumps [mini_magick](https://github.com/minimagick/minimagick) from 4.8.0 to 4.9.4.
- [Release notes](https://github.com/minimagick/minimagick/releases)
- [Commits](minimagick/minimagick@v4.8.0...v4.9.4)

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

Gemfile.lock

@@ -101,7 +101,7 @@ GEM
       rb-inotify (~> 0.9, >= 0.9.7)
       ruby_dep (~> 1.2)
     mercenary (0.3.6)
-    mini_magick (4.8.0)
+    mini_magick (4.9.4)
     mini_portile2 (2.3.0)
     minitest (5.11.3)
     nokogiri (1.8.4)
@@ -153,7 +153,7 @@ DEPENDENCIES
   jekyll-theme-pirati (= 7.0.0)
   json
   mini_magick
-  sprockets (~> 4.0.0.beta)
+  sprockets (= 4.0.0.beta8)
   tzinfo-data
   uglifier (~> 4.0.0)