Odoo: force postgres version in pg docker command line

The odoo docker image is based on debian:jessie, which only has
postgresql-client-9.4, while the official postgres docker
is 9.5. This causes a version mismatch and errors when using the
postgres command-line tools, such as `pg_dump`.

Fixes odoo/odoo#12275
Fixes odoo/docker#49
Fixes odoo/docker#54
Fixes odoo/docker#59

odoo/content.md

@@ -13,7 +13,7 @@ This image requires a running PostgreSQL server.
 ## Start a PostgreSQL server
 
 ```console
-$ docker run -d -e POSTGRES_USER=odoo -e POSTGRES_PASSWORD=odoo --name db postgres
+$ docker run -d -e POSTGRES_USER=odoo -e POSTGRES_PASSWORD=odoo --name db postgres:9.4
 ```
 
 ## Start an Odoo instance