Stable TableMigration.Reversible.toString (#79)

Fixed TableMigration Reversible class to have an explicit toString in
order to be sure that description comparisons in the flyway extension
work as expected.

src/main/scala/slick/migration/api/TableMigration.scala

@@ -84,6 +84,7 @@ object TableMigration {
         }
       }
     }
+    override def toString = underlying.toString
   }
 
   implicit def toReversible[T <: JdbcProfile#Table[_]]: ToReversible[TableMigration[T, Action.Reversible]] =