Release 6.0.0

This should have been release 5.0.0, but version 5.0.0 was published on
npm prematurely by a human mistake. This release requires a major bump,
as the underlying Acorn package has been bumped to ^7.0.0 with a
different default ECMAScript version.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "acorn-globals",
-  "version": "4.3.4",
+  "version": "6.0.0",
   "description": "Detect global variables in JavaScript using acorn",
   "keywords": [
     "ast",