Trying to create a hstore column in a create_table call works fine, while trying it in change_table produces the following error:
An error has occurred, this and all later migrations canceled:
undefined method `hstore' for #<ActiveRecord::ConnectionAdapters::Table:0x007fccd3d532a0>
This is on 4.0.0.beta1.
I confirmed this problam and I am working on a PR to fix it. This does not only affect hstore but all pg specific column types (json, xml, etc...)