fix(Changex.Formatter.Markdown): use Changex.Formatter.current_version

Previously, when running `mix changex.diff` the output would use version
specified in `mix.exs` - this would result in inconsistent results when
comparing with git tags. The `current_version` function that is included
when `use Changex.Formatter` is called is now used instead. The test has
also been updated to reflect this change.

Author: Gary Rennie

lib/changex/formatter/markdown.ex

@@ -1,4 +1,5 @@
 defmodule Changex.Formatter.Markdown do
+  use Changex.Formatter
 
   @moduledoc """
   Format changelog to the terminal in markdown format.
@@ -46,7 +47,7 @@ defmodule Changex.Formatter.Markdown do
   end
 
   defp heading(version) do
-    "# #{(version || Keyword.get(Mix.Project.config, :version))}\n"
+    "# #{(version || current_version)}\n"
   end
 
   defp types(commits, opts) do

test/changex/formatter/markdown_test.exs

@@ -24,7 +24,7 @@ defmodule Changex.Formatter.MarkdownTest do
 
   test "Formatting with an implicit version", %{commits: commits} do
     version = Keyword.get(Mix.Project.config, :version)
-    assert Changex.Formatter.Markdown.format(commits) == expected_markdown(version)
+    assert Changex.Formatter.Markdown.format(commits) == expected_markdown("v#{version}")
   end
 
   test "Formatting with a github url", %{commits: commits} do