fix formatter to not raise on empty header, update build image

Author: mattip

.circleci/config.yml

@@ -2,7 +2,7 @@ version: 2
 jobs:
   build:
     docker:
-      - image: circleci/ruby:2.5.1-node
+      - image: cimg/ruby2.6-node
     steps:
       - run:
           name: Checkout

_devel/formatter.py

@@ -17,7 +17,12 @@
         else:
             markdown += [line]
 
-    header = yaml.load(''.join(header))
+    header = yaml.load(''.join(header), Loader=yaml.BaseLoader)
+    if header is None:
+        # This assumes the markdown document has a yaml header
+        # but some documents, like the README.md do not
+        # Don't bother rendering them
+        exit()
 
     images = []
     try: