Use covertool plugin instead of deprecated rebar_covertool

rebar.config

@@ -52,8 +52,6 @@
    ]},
   {test,
    [
-    %% FIXME: Uncomment the below line when rebar_covertool-v1.3.0 is released
-    %% {plugins, [rebar_covertool]}
-    {plugins, [{rebar_covertool, {git, "https://github.com/covertool/covertool.git", {branch, "master"}}}]}
+    {plugins, [covertool]}
    ]}
  ]}.