Add current requirements for node versions

Right now we can only run on node versions below 10.x thanks to scrypt
dependencies.

Signed-off-by: Sheogorath <[email protected]>

Author: SISheogorath

package.json

@@ -132,7 +132,7 @@
     "xss": "^0.3.3"
   },
   "engines": {
-    "node": ">=6.x"
+    "node": ">=6.x <10.x"
   },
   "bugs": "https://github.com/hackmdio/hackmd/issues",
   "keywords": [