Bump minitest from v5.13 to v5.15

The latest version of minitest bumps the ruby version to include 3.0.

Author: ksylvest

Gemfile.lock

@@ -143,7 +143,7 @@ GEM
     mimemagic (0.3.3)
     mini_mime (1.0.2)
     mini_portile2 (2.4.0)
-    minitest (5.13.0)
+    minitest (5.14.2)
     nenv (0.3.0)
     nio4r (2.5.2)
     nokogiri (1.10.8)