Lock Rails 5.2 version to 5.2.7 on Ruby 2.2
Lock Rails 5.2 version to 5.2.7 on Ruby 2.2

The latest released version 5.2.7.1 is not compatible with Ruby 2.2

Revert this commit when a new 5.2 version will be released

Ref: rails/rails#44966
ifad/chronomodel