Upgrade minitest from 5.20.0 to 5.21.1 to fix mutex_m deprecation

Author: Dorian Marié

Gemfile.lock

@@ -327,7 +327,7 @@ GEM
     mini_magick (4.12.0)
     mini_mime (1.1.5)
     mini_portile2 (2.8.5)
-    minitest (5.20.0)
+    minitest (5.21.1)
     minitest-bisect (1.7.0)
       minitest-server (~> 1.0)
       path_expander (~> 1.1)