Specify Rails version in CarrierWave migration
Specify Rails version in CarrierWave migration

[This build][build] failed because CarrierWave didn't specify the Rails
version when it generated this migration file. There might have been a
deprecation warning, but I didn't see it. Anyway, this commit simply
adds the Rails version to that migration.

[build]: https://circleci.com/gh/artsy/bearden/459

artsy/bearden,artsy/bearden,artsy/bearden,artsy/bearden