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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

This MR adds a string (thus max 255 chars) field to the enviroments
table to expose it later in other features.

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4093#note_12571602
and
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4093#note_12571670

This reverts commit 13e37a3e.

Using update_column to store the boolean flag to avoid
any side effects with the current state of the project
instance