lock down dev dependencies

Let's be very explicit about updating these. They are very critical to
the build process, so we should always read the changelogs / commit log
before upgrading.

package.json

@@ -54,12 +54,12 @@
     "cross-zip": "^2.0.1",
     "depcheck": "^0.6.4",
     "electron": "1.4.15",
-    "electron-osx-sign": "^0.4.3",
-    "electron-packager": "^8.0.0",
-    "electron-winstaller": "^2.3.0",
+    "electron-osx-sign": "0.4.3",
+    "electron-packager": "~8.5.1",
+    "electron-winstaller": "~2.5.2",
     "gh-release": "^2.0.3",
     "minimist": "^1.2.0",
-    "nobin-debian-installer": "^0.0.10",
+    "nobin-debian-installer": "0.0.10",
     "nodemon": "^1.10.2",
     "open": "0.0.5",
     "plist": "^2.0.1",