sudo: false
language: ruby
install: gem install minitest timecop coveralls
rvm:
  - 2.0.0
  - 2.1.9
  - 2.2.8
  - 2.3.5
  - 2.4.2
  - 2.5.0
script: rake test

sudo: false
language: ruby
install: gem install minitest timecop coveralls
rvm:
  - 2.3.5
  - 2.4.2
  - 2.5.1
script: rake test

Drop ruby < 2.3 support
Drop ruby < 2.3 support

in order to create mutable strings with String#+@
