all: use stretch instead of buster for all builds (#1768)

Change the Debian version that the Go 1.13 checks, tests, and publishing occurs with to Debian Stretch, instead of Debian Buster.

When we added tests for Go 1.13 the only image available on DockerHub was based on Debian Stretch. We made a case for publishing a Stretch version for at least a year while Stretch is still in use in docker-library/golang#296 and it was added in docker-library/golang/pull/300 and docker-library/official-images#6597.

Since we're still using Debian Stretch we agreed it would be worthwhile to run our tests and builds on it until we upgrade.

I left the postgresql fallback install code that pulls the deb from the official postgres apt repo when it's not available in the default system repo. That was also added for Buster because Buster doesn't have the version we need, version 9.6. I left that code there because we're likely to use it soon enough when we move everything to Buster anyway.

Close #1718

.circleci/config.yml

@@ -191,7 +191,7 @@ jobs:
   check_code_1_13:
     working_directory: /go/src/github.com/stellar/go
     docker:
-      - image: circleci/golang:1.13-buster
+      - image: circleci/golang:1.13-stretch
     steps:
       - install_go_deps
       - check_go_deps
@@ -223,7 +223,7 @@ jobs:
   test_code_1_13:
     working_directory: /go/src/github.com/stellar/go
     docker:
-      - image: circleci/golang:1.13-buster
+      - image: circleci/golang:1.13-stretch
         environment:
           GO111MODULE: "on"
           PGHOST: localhost
@@ -247,7 +247,7 @@ jobs:
   publish_artifacts:
     working_directory: /go/src/github.com/stellar/go
     docker:
-      - image: circleci/golang:1.13-buster
+      - image: circleci/golang:1.13-stretch
     steps:
       - check_deprecations
       - install_go_deps
@@ -271,7 +271,7 @@ jobs:
   ingest_state_diff:
     working_directory: /go/src/github.com/stellar/go/
     docker:
-      - image: circleci/golang:1.13-buster
+      - image: circleci/golang:1.13-stretch
         environment:
           PGHOST: localhost
           PGPORT: 5432