remove rubyversion and change parallel test setup

.ruby-version

@@ -1 +0,0 @@
-2.6.1

Gemfile

@@ -17,6 +17,7 @@ group :test do
   gem "minitest-color"
   gem 'minitest-fail-fast'
   gem 'minitest-profile'
+  gem "minitest-parallel_fork" , require: false
 end
 
 group :development do

test/test_all.rb

@@ -1,7 +1,5 @@
 ENV["TEST_ALL"] = "true"
 require_relative "helper"
+require "minitest/parallel_fork" if ENV["NCPU"]
 
-Dir["test/**/test_*.rb"].each { |f|
-  next if f.include?("stash/")
-  require_relative "../#{f}"
-}
+Dir["test/**/test_*.rb"].each { |f|  require_relative "../#{f}" }