add prepublish hook

Author: miafan23

package-lock.json

@@ -1,6 +1,6 @@
 {
   "name": "fontnik",
-  "version": "0.7.4-dev.2",
+  "version": "0.7.4-dev.3",
   "description": "A library that delivers a range of glyphs rendered as SDFs (signed distance fields) in a protobuf.",
   "keywords": [
     "font",
@@ -51,6 +51,7 @@
     "test": "./node_modules/.bin/tape test/**/*.test.js",
     "prebuildify": "prebuildify --napi --tag-uv --tag-libc --strip"
   },
+  "prepublishOnly": "npm run download-binaries",
   "binary": {
     "module_name": "fontnik",
     "module_path": "./lib/binding/"