withoutEqual should be false for undoing the migration.
This will never resturn the last run migration.
also  withoutEquals: false seems redundant, it's not being used anywhere.
 
  Migration.prototype.isBefore = function(dateString, options) {
    options = Utils._.extend({
      withoutEquals: false
    }, options || {})

    var date = Migration.parseFilename(dateString.toString() + '-foo.js').date

    return options.withoutEqual ? (date > this.date) : (date >= this.date)
  }