Require minimum Ruby 2.2 version

[ArturT]

Related to #112

minitest 4 is bundled with Ruby 2.1

Forcing to use minitest 5 with Ruby 2.1 still causes CI build to fail https://github.com/KnapsackPro/knapsack/runs/2883581100?check_suite_focus=true

KNAPSACK_TEST_FILE_PATTERN="test_examples/**{,/*/**}/*_test.rb" bundle exec rake "knapsack:minitest[--verbose]"

...

/opt/hostedtoolcache/Ruby/2.1.9/x64/lib/ruby/2.1.0/minitest/unit.rb:26:in `const_missing': uninitialized constant MiniTest::Test (NameError)
	from /home/runner/work/knapsack/knapsack/lib/knapsack/adapters/minitest_adapter.rb:24:in `bind_time_tracker'

Related

Support for Minitest 4 was removed in #114

solution

Let's use Ruby 2.2 as the minimum required version.