Minor build chores for whitespace and lockfile (#366)

* chore: convert spaces to tabs in Gruntfile

Align the file with the EditorConfig setting

* chore: ignore package-lock.json

This file is produced by NPM 5, but this repo didn't use the old
shrinkwrap version so this should be excluded

.gitignore

@@ -8,4 +8,5 @@ axe.*.js
 *.tgz
 npm-shrinkwrap.json
 typings/axe-core/axe-core-tests.js
-doc/rule-descriptions.*.md
+doc/rule-descriptions.*.md
+package-lock.json

Gruntfile.js

@@ -54,14 +54,14 @@ module.exports = function (grunt) {
 				]
 			}
 		},
-    retire: {
+	retire: {
 			options: {
 				/** list of files to ignore **/
 				ignorefile: '.retireignore.json' //or '.retireignore.json'
 			},
 			js: ['lib/*.js'], /** Which js-files to scan. **/
 			node: ['./'] /** Which node directories to scan (containing package.json). **/
-    },
+	},
 		clean: ['dist', 'tmp', 'axe.js', 'axe.*.js'],
 		babel: {
 			options: {