Change GitHub upload to use JS GitHub lib

Needed for Appveyor when running releases

script/upload-to-github.js

@@ -0,0 +1,21 @@
+const GitHub = require('github')
+const github = new GitHub()
+github.authenticate({type: 'token', token: process.env.ELECTRON_GITHUB_TOKEN})
+
+let filePath = process.argv[2]
+let fileName = process.argv[3]
+let releaseId = process.argv[4]
+
+let githubOpts = {
+  owner: 'electron',
+  repo: 'electron',
+  id: releaseId,
+  filePath: filePath,
+  name: fileName
+}
+github.repos.uploadAsset(githubOpts).then(() => {
+  process.exit()
+}).catch((err) => {
+  console.log(`Error uploading ${fileName} to GitHub:`, err)
+  process.exitCode = 1
+})