Update CI for new Elixir

[greg-rychlewski]

To make my ramblings in the other PR more concrete:

1. Remove Postgres 9.5 from the CI. We need a special pinned version of the client tools to use it and they are no longer supported by Alpine
2. Slightly loosen the assertion for a mysql dump test. This is because the new Alpine seems to default to using mariadb dump and they include the column names in the insert statement. To make the test work with either dump i just loosened the assertion a bit. I tried seeing if there was a nice way to remove the mariadb dump and use the mysql one but it doesn't seem possible without complicating the CI