Bump markdown to 2.1.5 to improve performance and include fixes (#2267)

Author: srawlins

pubspec.yaml

@@ -18,7 +18,7 @@ dependencies:
   http_parser: '>=3.0.3 <4.0.0'
   logging: ^0.11.3+1
   process: ^3.0.5
-  markdown: ^2.1.1
+  markdown: ^2.1.5
   meta: ^1.1.8
   mustache: ^1.1.0
   package_config: '>=0.1.5 <2.0.0'