After removing ws, node version 10 should work

In my local environment I switched to Fedora 29. Fedora 29 comes with
NodeJS version 10.

As far as I can say, it works, so let's try to remove the restriction to
"<10.x"

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

package.json

@@ -135,7 +135,7 @@
     "xss": "^1.0.3"
   },
   "engines": {
-    "node": ">=6.x <10.x"
+    "node": ">=6.x"
   },
   "bugs": "https://github.com/hackmdio/codimd/issues",
   "keywords": [