build

Use calling migration version for `safe_version?` when calling `revert` #233

Job	Run time
build (3.2, Gemfile)	1m 19s
build (3.1, gemfiles/activerecord70.gemfile)	1m 27s
build (3.0, gemfiles/activerecord61.gemfile)	1m 10s
build (2.7, gemfiles/activerecord60.gemfile)	1m 54s
build (2.6, gemfiles/activerecord52.gemfile)	1m 48s
	7m 38s

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