When you initiate replication between two PostgreSQL databases on Cloud 66, we setup streaming replication between the master and replica servers. Streaming replication is based on log shipping between servers, which generally isn't possible between two servers running vastly different versions of PostgreSQL.
As such, we cannot establish replication between servers running different major release levels (e.g. 8.4 and 9.3). Though running replication between different minor release levels (e.g. 9.3 and 9.4) should work (because PostgreSQL has a policy not to make changes to disk formats between minor releases), there are also cases where this won't work.
For example, if you setup replication between a master (on 9.3) and a replica (9.4), you may see this error on the replica server:
FATAL: database files are incompatible with server DETAIL: The data directory was initialized by PostgreSQL version 9.3, which is not compatible with this version 9.4.x.
In this case, you need to upgrade the data and libraries of the master server (9.3) with pg_upgrade before starting the replication.
Remember that you can set the version of PostgreSQL to install for your application by using a manifest file, like so:
production: postgresql: configuration: version: 9.3.6