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executables: Reject Lua 5.1 early Prevents attempting to load libraries that may no longer be found and crashing with a traceback. Platforms like Debian where multiple Lua versions can be installed at the same time and 'lua' pointing to one of the installed interpreters via symlinks, there's the possibility that prosody/prosodyctl may be invoked with Lua 5.1, which will no longer have any of the rest of Prosody libraries available to be require(), and thus would immediately fail with an unfriendly traceback. Checking and aborting early with a friendlier message and reference to more information is better. Part of #1600

Welcome hackers!

This project accepts and *encourages* contributions. If you would like to get 
involved you can join us on our mailing list and discussion rooms. More 
information on these at https://prosody.im/discuss

Patches are welcome, though before sending we would appreciate if you read 
docs/coding_style.md for guidelines on how to format your code, and other tips.

Documentation for developers can be found at https://prosody.im/doc/developers

Have fun :)