add .ruby-version and .ruby-gemset for development.
remove spec.add_development_dependency "minitest" to avoid following warnings:

    MiniTest::Unit::TestCase is now Minitest::Test.

minitest 5 rename MiniTest::Unit::TestCase to Minitest::Test.
But use minitest 4 that ruby bunled for this gem.

If minitest 5 is already installed, uninstall by following commands:

    gem list | grep minitest
    gem uninstall minitest