v2.3.1

Changes:

• 👨‍💻 Improve createIndex helper
• 👨‍💻 Improve error output when FileSystem destination not writable; Related to #857, thanks to @Leekao
• 🐞 Fix custom allowedOrigins option for CORS; Closing #850, thanks to @djlogan2;
• 📔 Improve AWS S3 documentation, by @xsyann;

Notes:

• 👨‍🔬 Tested with latest release of meteor@2.8.0