circleci: Add explict version to `setup_remote_docker`. NFC. (#646)

Apparently we are supposed to do this:
 https://discuss.circleci.com/t/old-linux-machine-image-remote-docker-deprecation/37572

.circleci/config.yml

diff --git a/.circleci/config.yml b/.circleci/config.yml
index 5d2a1f5..7994d6d 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -100,7 +100,8 @@
       - run:
           name: install docker
           command: apt-get update -q && apt-get install -q -y docker.io
-      - setup_remote_docker
+      - setup_remote_docker:
+          version: 19.03.13
       # Build and test the tip-of-tree build of EMSDK
       - run:
           name: build
@@ -116,7 +117,8 @@
       - run:
           name: install docker
           command: apt-get update -q && apt-get install -q -y docker.io
-      - setup_remote_docker
+      - setup_remote_docker:
+          version: 19.03.13
       - run:
           name: build
           command: make -C ./docker version=${CIRCLE_TAG} build