irb 1.6.0 on Ruby Gems

What's Changed

Make sure ls doesn't return an array by @k0kubun in #461
Use class methods of File over Kernel.open by @nobu in #462
Test debug commands without yamatanooroti by @st0012 in #464
Use the proper ruby command and library path by @nobu in #465
Disable debug cmd tests based on project structure instead of env by @st0012 in #466
Require pathname in test helper by @st0012 in #467
Add workflow for testing IRB against Ruby core by @st0012 in #468
Close leaked pty IOs by @nobu in #470
Allow disabling autocompletion with IRB_USE_AUTOCOMPLETE=false by @st0012 in #469
Lazily load the multi-irb extension by @st0012 in #472
Add show_cmds command by @st0012 in #463
Kill PTY process after test is finished by @st0012 in #471
Gracefully handle missing command argument by @st0012 in #473
Add show_doc as an alias to the help command by @st0012 in #475
Update readme's commands section by @st0012 in #476

Full Changelog: v1.5.1...v1.6.0