Fix Windows npm install

Like `appdmg`, `electron-installer-debian` cannot be installed on Windows.

package.json

@@ -55,7 +55,6 @@
     "cross-zip": "^2.1.6",
     "depcheck": "^0.8.3",
     "electron": "~6.0.9",
-    "electron-installer-debian": "^2.0.0",
     "electron-osx-sign": "^0.4.13",
     "electron-packager": "^14.0.6",
     "electron-winstaller": "^4.0.0",
@@ -88,7 +87,8 @@
   "license": "MIT",
   "main": "index.js",
   "optionalDependencies": {
-    "appdmg": "^0.6.0"
+    "appdmg": "^0.6.0",
+    "electron-installer-debian": "^2.0.0"
   },
   "private": true,
   "productName": "WebTorrent",