Update Earmark (#1671)

It contains security fixes. Albeit they are unlikely to
apply to ExDoc, it doesn't hurt to update.

Author: RobertDober

mix.exs

@@ -31,7 +31,7 @@ defmodule ExDoc.Mixfile do
 
   defp deps do
     [
-      {:earmark_parser, "~> 1.4.19"},
+      {:earmark_parser, "~> 1.4.31"},
       {:makeup_elixir, "~> 0.14"},
       {:makeup_erlang, "~> 0.1"},
       {:makeup_html, ">= 0.0.0", only: :dev},

test/ex_doc/doc_ast_test.exs

@@ -67,7 +67,7 @@ defmodule ExDoc.DocASTTest do
       """
 
       ast = DocAST.parse!(markdown, "text/markdown")
-      assert DocAST.to_string(ast) == ~s{<span><i></i><i></i></span>}
+      assert DocAST.to_string(ast) == ~s{<span><i></i>\n<i></i></span>}
     end
 
     test "with fun" do