Remove unused minitest from Gemfile (#510)

Author: byroot

Gemfile

@@ -6,7 +6,6 @@ gem "benchmark_driver"
 gem "fiddle", platform: :ruby
 gem "rake", ">= 12.3.3"
 gem "rake-compiler", ">= 0.9"
-gem "minitest", "< 5.0.0"
 gem "irb"
 gem "test-unit"
 gem "test-unit-ruby-core"