update ruby gem and format by rubocop

[codecov[bot]]

Codecov Report

Merging #55 (8744d34) into master (f6e01c8) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #55   +/-   ##
=======================================
  Coverage   98.03%   98.03%           
=======================================
  Files          18       18           
  Lines         153      153           
=======================================
  Hits          150      150           
  Misses          3        3           

Impacted Files	Coverage Δ
lib/rubiderella/errors.rb	100.00% <ø> (ø)
lib/rubiderella/person.rb	100.00% <ø> (ø)
lib/rubiderella/person/birthday.rb	100.00% <100.00%> (ø)

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[hikko624]

unsupport ruby 2.4