fix: backward compatibility issues

use concat and reduce instead of flat/map

Author: os-moussao

docker-compose.yaml

@@ -0,0 +1,7 @@
+services:
+  node10:
+    image: node:10
+    volumes:
+      - .:/app
+    working_dir: /app
+    command: npm run test

src/lib/isDuration.js

@@ -34,9 +34,8 @@ const BaseDurationUnits = [
   'Ms',
 ];
 
-
-const AllDurationUnits = new Set(BaseDurationUnits.map(unit => [unit,
-  unit.toUpperCase(), unit.toLowerCase()]).flat());
+const AllDurationUnits = new Set(BaseDurationUnits.reduce((acc, unit) => acc.concat([
+  unit, unit.toUpperCase(), unit.toLowerCase()]), []));
 
 /**
  * Checks if the string is a valid duration.