[CI] Test against Ruby 2.3, 2.4 and 2.5 (#476)
[CI] Test against Ruby 2.3, 2.4 and 2.5 (#476)

* [CI] Test against Ruby 2.3, 2.4 and 2.5

* [CI] Remove unnecessary bundler install

meew0/discordrb,meew0/discordrb