Match circleci test suite postgres version with closest match to current RDS version
Match circleci test suite postgres version with closest match to current RDS version

RDS postgres uses Amazon Linux distro 9.6.20 at time of writing

There appears to be suttle differences in ordering/collation
which have cause bugs, indirectly, in fee calculation.

Regardless it is better to use a version of postgres that
most closely matches an RDS postgres instances and OS.
The debian hosted versions available as convenience images
from [circleci dockerhub](https://circleci.com/docs/2.0/docker-image-tags.json)
would seem to be as close to this as possible while being reliably available.

Comparison of differen postgres versions below:

example postgres versions for different images/OSes
 - local postgres install
```
[{"version"=>"PostgreSQL 9.6.21 on x86_64-apple-darwin19.6.0, compiled by Apple clang version 12.0.0 (clang-1200.0.32.29), 64-bit"}]
```
 - circleci using image `circleci/postgres:9.6.15`
```
[{"version"=>"PostgreSQL 9.6.15 on x86_64-pc-linux-gnu (Debian 9.6.15-1.pgdg90+1), compiled by gcc (Debian 6.3.0-18+deb9u1) 6.3.0 20170516, 64-bit"}]
```

 - circleci using image `circleci/postgres:9.6.15-alpine`
```
[{"version"=>"PostgreSQL 9.6.15 on x86_64-pc-linux-musl, compiled by gcc (Alpine 8.3.0) 8.3.0, 64-bit"}]
```
 - RDS postgres - `SELECT version();`
```
 PostgreSQL 9.6.20 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-11), 64-bit
```

ministryofjustice/advocate-defence-payments,ministryofjustice/advocate-defence-payments,ministryofjustice/advocate-defence-payments,ministryofjustice/advocate-defence-payments