explicitly cast column types when migrating

PG doesn't support implicit casting anymore because it could incur data
loss.  This explicitly casts data types using the API in upstream Rails.

This depends on manageiq/rails#15
Bump to rails fork containing support for casting.

connection.change_column :strings, :somedate, :timestamp, cast_as: :timestamp
connection.change_column :strings, :somedate, :timestamp, using: 'CAST("somedate" AS timestamp)'
Merge pull request #1156 from tenderlove/explicit_cast

explicitly cast column types when migrating