Change wording on reference manual for better compatibility

["Soni L." <fakedme@...>]

Currently _VERSION states:

A global variable (not a function) that holds a string containing the 
current interpreter version. The current value of this variable is "|Lua 
5.3|".

It would be better, for compatibility purposes, to put it as:

A global variable (not a function) that holds a string containing the 
current Lua version. The current value of this variable is "|Lua 5.3|".

That way a script can safely assume _VERSION contains the Lua version, 
or tell if an implementation deviates from standard Lua. _VERSION == 
"Lua 5.3" would be true in Lua 5.3, and any other Lua 5.3 
implementation, but if a fork of Lua 5.3 appeared, it would be 
encouraged to change _VERSION from "Lua 5.3" to (e.g.) "LuaFork 1.0".

This would fix LuaJ being stubborn [1].

[1] https://sourceforge.net/p/luaj/bugs/27/

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