bundle update --conservative minitest

`bundle install` on ruby 3.0 makes following error

```
minitest-5.14.1 requires ruby version ~> 2.2, which is incompatible with the current version, ruby 3.0.0p0
```

And minitest 5.14.4 still supports ruby 2.2

https://rubygems.org/gems/minitest/versions/5.14.4

Gemfile.lock

@@ -22,7 +22,7 @@ GEM
     diff-lcs (1.3)
     i18n (1.8.3)
       concurrent-ruby (~> 1.0)
-    minitest (5.14.1)
+    minitest (5.14.4)
     mysql2 (0.5.3)
     rake (13.0.1)
     rspec (3.6.0)
@@ -67,4 +67,4 @@ DEPENDENCIES
   sqlite3
 
 BUNDLED WITH
-   2.1.4
+   2.2.3