Bump minitest from 5.18.1 to 5.20.0

Bumps [minitest](https://github.com/minitest/minitest) from 5.18.1 to 5.20.0.
- [Changelog](https://github.com/minitest/minitest/blob/master/History.rdoc)
- [Commits](minitest/minitest@v5.18.1...v5.20.0)

---
updated-dependencies:
- dependency-name: minitest
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Gemfile

@@ -50,7 +50,7 @@ group :test do
   # Disallow v5.19 for now since it breaks mocha.
   # See: https://github.com/freerange/mocha/issues/614
   # Remove this line when mocha has fixed the issue
-  gem "minitest", "< 5.19"
+  gem "minitest", "< 5.21"
 end
 
 # development dependencies

Gemfile.lock

@@ -112,7 +112,7 @@ GEM
     method_source (1.0.0)
     mini_mime (1.1.2)
     mini_portile2 (2.8.1)
-    minitest (5.18.1)
+    minitest (5.20.0)
     mocha (2.1.0)
       ruby2_keywords (>= 0.0.5)
     msgpack (1.7.0)
@@ -257,7 +257,7 @@ DEPENDENCIES
   listen (~> 3.0)
   lookbook (~> 2.1.1)
   matrix (~> 0.4.2)
-  minitest (< 5.19)
+  minitest (< 5.21)
   mocha
   primer_view_components!
   pry