This cop will analyze migrations that add columns with string, and
report an offense if the string has no limit enforced
Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/64505
DB schema generated by a migration may look different in
rails 4 and 5 (because rails 5 may use different default values).
For this reason it's important to explicitly set for which rails
version a migration was written for.
See https://stackoverflow.com/questions/35929869/activerecordmigration-deprecation-warning-asks-for-rails-version-but-im-no/35930912#35930912