Update RuboCop settings

[gemmaro]

Changes:

• Update RuboCop and config files
• Add RuobCop extensions for Rake and RSpec
• Minimal Ruby version is now 2.7
• Apply RuboCop safe rules, except for unsafe rules

Memo:

.travis.yml says 2.3.0, so it might be the version.

Gem depends seems to require at least 2.4.0:

$ ruby -v            
ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin21]

$ bundle
Fetching gem metadata from https://rubygems.org/...........
Resolving dependencies...
listen-3.7.1 requires ruby version >= 2.4.0, which is incompatible with the current version, ruby 2.3.0p0

[iyuuya]