Support npm version 6.x (#1866)

Summary:
**Summary**

Now `nvm install 8` ends up installing npm 6.4.1 but our `package.json` doesn't support that npm version. This PR includes 6.x in the versions we support.

More context: facebookarchive/draft-js#1864 (comment)

**Test Plan**

CI tests.
Pull Request resolved: facebookarchive/draft-js#1866

Reviewed By: niveditc

Differential Revision: D9789746

Pulled By: niveditc

fbshipit-source-id: 8bdf8573c84d66bdce8281c4b2660df953399583

Author: aforismesen

package.json

@@ -86,7 +86,7 @@
   },
   "devEngines": {
     "node": "8.x || 9.x",
-    "npm": "2.x || 3.x || 5.x"
+    "npm": "2.x || 3.x || 5.x || 6.x"
   },
   "jest": {
     "automock": true,