Support Rails 6.1

[chocoby]

• Rails 6.1 をサポート
• README: 開発環境の Gem のインストールや RSpec の実行方法についてコマンドを変更
• RSpec を実行する Rake タスクを削除
• Ruby 2.3 において SimpleCov のエラーが発生するため、ビルド対象から削除

/opt/hostedtoolcache/Ruby/2.3.8/x64/lib/ruby/gems/2.3.0/gems/simplecov-lcov-0.8.0/lib/simplecov-lcov.rb:90:in `format_file': undefined method `branch_coverage?' for SimpleCov:Module (NoMethodError)
	from /opt/hostedtoolcache/Ruby/2.3.8/x64/lib/ruby/gems/2.3.0/gems/simplecov-lcov-0.8.0/lib/simplecov-lcov.rb:75:in `block (2 levels) in write_lcov_to_single_file!'
	from /opt/hostedtoolcache/Ruby/2.3.8/x64/lib/ruby/gems/2.3.0/gems/simplecov-lcov-0.8.0/lib/simplecov-lcov.rb:75:in `each'
	from /opt/hostedtoolcache/Ruby/2.3.8/x64/lib/ruby/gems/2.3.0/gems/simplecov-lcov-0.8.0/lib/simplecov-lcov.rb:75:in `block in write_lcov_to_single_file!'
	from /opt/hostedtoolcache/Ruby/2.3.8/x64/lib/ruby/gems/2.3.0/gems/simplecov-lcov-0.8.0/lib/simplecov-lcov.rb:74:in `open'
	from /opt/hostedtoolcache/Ruby/2.3.8/x64/lib/ruby/gems/2.3.0/gems/simplecov-lcov-0.8.0/lib/simplecov-lcov.rb:74:in `write_lcov_to_single_file!'
	from /opt/hostedtoolcache/Ruby/2.3.8/x64/lib/ruby/gems/2.3.0/gems/simplecov-lcov-0.8.0/lib/simplecov-lcov.rb:18:in `format'
	from /opt/hostedtoolcache/Ruby/2.3.8/x64/lib/ruby/gems/2.3.0/gems/simplecov-0.17.1/lib/simplecov/result.rb:48:in `format!'
	from /opt/hostedtoolcache/Ruby/2.3.8/x64/lib/ruby/gems/2.3.0/gems/simplecov-0.17.1/lib/simplecov/configuration.rb:182:in `block in at_exit'
	from /opt/hostedtoolcache/Ruby/2.3.8/x64/lib/ruby/gems/2.3.0/gems/simplecov-0.17.1/lib/simplecov.rb:201:in `run_exit_tasks!'
	from /opt/hostedtoolcache/Ruby/2.3.8/x64/lib/ruby/gems/2.3.0/gems/simplecov-0.17.1/lib/simplecov/defaults.rb:29:in `block in <top (required)>'

[coveralls]

Coverage remained the same at 100.0% when pulling 7b33d9d on support-rails61 into 1f93bc0 on master.