build-unit-test-docker: fmt 5.3.0 -> 6.1.2

Change-Id: I8aa6a01770ab919efe38e930a952c4e6f74a6814
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index e548c18..cebf0ef 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -95,7 +95,7 @@
   [cereal]=v1.2.2
   [catch2]=v2.11.1
   [CLI11]=v1.9.0
-  [fmt]=5.3.0
+  [fmt]=6.1.2
   # Snapshot from 2019-03-10
   [function2]=e3695b4b4fa3c672e25c6462d7900f8d2417a417
   # Snapshot from 2018-12-17