add prebuild support for Electron 9 betas (#252)

Author: shiftkey

package.json

@@ -36,7 +36,7 @@
     "test": "npm run lint && npm build . && mocha --require babel-core/register spec/",
     "prebuild-node": "prebuild -t 8.9.0 -t 9.4.0 -t 10.11.0 -t 11.9.0 -t 12.0.0 -t 13.0.0 --strip",
     "prebuild-node-ia32": "prebuild -t 8.9.0 -t 9.4.0 -a ia32 --strip",
-    "prebuild-electron": "prebuild -t 5.0.0 -t 6.0.0 -t 7.0.0 -t 8.0.0 -r electron --strip",
+    "prebuild-electron": "prebuild -t 5.0.0 -t 6.0.0 -t 7.0.0 -t 8.0.0 -t 9.0.0-beta.2 -r electron --strip",
     "prebuild-electron-ia32": "prebuild -t 5.0.0 -t 6.0.0 -t 7.0.0 -t 8.0.0 -r electron -a ia32 --strip",
     "upload": "node ./script/upload.js",
     "postpublish": "git push --follow-tags"