About time to have a proper release version

Supported rubies (as of the github ci workflow):

• 3.0.4
• 3.1.2
• 3.2.0

Mainly cleaning up the gemspec file and fixing offenses.

Version 0.0.3

Remove support for eol rubies, supported rubies are now

• 2.4.5
• 2.5.3
• 2.6.0
• jruby-9.1
• jruby-9.2

Replaced deprecated BigDecimal.new with BigDecimal!

Furthermore fixes for a lot of rubocop offenses.