Exclude ruby-head and ruby-debug until minitest allows Ruby 3.1

yahonda · yahonda · commit d0ef6caf21ae · 2020-12-30T17:08:13.000+09:00
Refer minitest/minitest#861 minitest/minitest#862
diff --git a/.travis.yml b/.travis.yml
@@ -43,7 +43,6 @@ rvm:
   - 2.6.6
   - 2.5.8
   - jruby-9.2.14.0
-  - ruby-head
   - jruby-head
 
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