Version 0.4.0

Welcome to 2021, where Ruby version 2.6 is the lowest with official support, Bundler is the default for managing packages and RSpec version 3 is used to test things. This version updates Groupie into this decade.

• Refactor: update Groupie to 2021 standards
• Feat: raise Groupie::Error instead of RuntimeError
• Feat: deprecate String#tokenize in favor of Groupie.tokenize
• Doc: document API of Groupie
• Doc: update readme with examples
• Refactor: reorder Groupie methods by importance
• Refactor: simplify Groupie#classify
• Refactor: reduce complexity of Groupie#unique_words
• Refactor: simplify Groupie#classify_text