Support markdown 3.0.0 (#2394)

Author: srawlins

pubspec.yaml

@@ -15,7 +15,7 @@ dependencies:
   glob: '>=1.1.2 <2.0.0'
   html: '>=0.12.1 <0.15.0'
   logging: ^0.11.3+1
-  markdown: ^2.1.5
+  markdown: '>=2.1.5 <4.0.0'
   meta: ^1.1.8
   mustache: ^1.1.0
   package_config: '>=0.1.5 <2.0.0'