Drop Node 8 support

As can be seen at https://github.com/nodejs/Release, Node 8 reached
"end" of life on 2019-12-31, and is no longer actively supported.

We will stop testing in Node 8 and start testing in Node 13, which will
become the next LTS release from April 2020.

.circleci/config.yml

@@ -46,15 +46,6 @@ jobs:
           name: lint
           command: npm run lint
 
-  node-8:
-    docker:
-      - image: circleci/node:8
-    steps:
-      - *attach-step
-      - run:
-          name: Test
-          command: npm test
-
   node-10:
     docker:
       - image: circleci/node:10
@@ -76,6 +67,15 @@ jobs:
           name: Upload coverage report
           command: bash <(curl -s https://codecov.io/bash) -F unit -s coverage/lcov.info
 
+  node-13:
+    docker:
+      - image: circleci/node:13
+    steps:
+      - *attach-step
+      - run:
+          name: Test
+          command: npm test
+
 workflows:
   version: 2
   referee:
@@ -84,12 +84,12 @@ workflows:
       - lint:
           requires:
             - install-dependencies
-      - node-8:
-          requires:
-            - install-dependencies
       - node-10:
           requires:
             - install-dependencies
       - node-12:
           requires:
             - install-dependencies
+      - node-13:
+          requires:
+            - install-dependencies