use Earmark.as_html! (#671)

Earmark.to_html was [deprecated as of v1.1](pragdave/earmark@8f8a05f).
This changes to use the new `as_html!` which prints any errors to STDERR automatically.

lib/ex_doc/markdown/earmark.ex

@@ -31,6 +31,6 @@ defmodule ExDoc.Markdown.Earmark do
              file: Keyword.get(opts, :file),
              breaks: Keyword.get(opts, :breaks, false),
              smartypants: Keyword.get(opts, :smartypants, true))
-    Earmark.to_html(text, options)
+    Earmark.as_html!(text, options)
   end
 end

mix.exs

@@ -21,7 +21,7 @@ defmodule ExDoc.Mixfile do
   end
 
   defp deps do
-    [{:earmark, "~> 1.0"},
+    [{:earmark, "~> 1.1"},
      {:markdown, github: "devinus/markdown", only: :test},
      {:cmark, "~> 0.5", only: :test},
      {:excoveralls, "~> 0.3", only: :test}]