You should script a bash script for this. I assume you always place the dumps in the same folder. Create a bash script that automaticly checks all files in the directory and let it remove the oldest (or for example dumps older than xx days)

A great example: https://wiki.postgresql.org/wiki/Automated_Backup_on_Linux 
Also have a look at this script: https://www.odoo.com/nl_NL/forum/help-1/question/how-do-i-automate-database-backups-8899
You can customize this as you wish. On a specific date, looping over them, creating a new one every xx time etc.

Update: If you also want to drop the db's / schema's from your postgreSQL you should look at the dropdb and drop schema commands. For example: http://www.postgresql.org/docs/9.4/static/sql-dropschema.html

Update #2

I've created a module which automatically does all this and is integrated in Odoo (V8).Have a look at my guide/topic about it here.