From the bundler website:
The specifier ~> has a special meaning, best shown by example:
'~> 2.0.3'is identical to'>= 2.0.3‘ and'< 2.1.'
'~> 2.1'is identical to'>= 2.1'and'< 3.0'.
'~> 2.2.beta'will match prerelease versions like'2.2.beta.12'.
See https://bundler.io/gemfile.html and http://guides.rubygems.org/patterns/#pessimistic-version-constraint