temp fix for Node.js internal stack traces showing

With Node.js 6 and up.

See: sindresorhus/clean-stack@e26cacd#diff-1dd241c4cd3fd1dd89c570cee98b79ddR50

lib/beautify-stack.js

@@ -2,6 +2,25 @@
 var StackUtils = require('stack-utils');
 var debug = require('debug')('ava');
 
+//-----------
+// embedded `clean-stack` module (because it requires Node.js 4)
+var extractPathRegex = /\s+at.*(?:\(|\s)(.*)\)?/;
+var pathRegex = /^(?:(?:(?:node|(?:internal\/[\w/]*|.*node_modules\/babel-polyfill\/.*)?\w+)\.js:\d+:\d+)|native)/;
+
+var cleanStack = function (stack) {
+	return stack.replace(/\\/g, '/').split('\n').filter(function (x) {
+		var pathMatches = x.match(extractPathRegex);
+		if (pathMatches === null || !pathMatches[1]) {
+			return true;
+		}
+
+		return !pathRegex.test(pathMatches[1]);
+	}).filter(function (x) {
+		return x.trim() !== '';
+	}).join('\n');
+};
+//-----------
+
 function indent(str) {
 	return '    ' + str;
 }
@@ -25,6 +44,10 @@ module.exports = function (stack) {
 		return '';
 	}
 
+	// workaround for https://github.com/tapjs/stack-utils/issues/14
+	// TODO: fix it in `stack-utils`
+	stack = cleanStack(stack);
+
 	var title = stack.split('\n')[0];
 	var lines = stackUtils
 		.clean(stack)