maint: add Lua 5.3 to README.md.

* README.md: Add Lua 5.3 reference.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>

README.md

@@ -6,8 +6,8 @@ By the [luaposix project][GitHub]
 [![travis-ci status](https://secure.travis-ci.org/luaposix/luaposix.png?branche=master)](http://travis-ci.org/luaposix/luaposix/builds)
 [![Stories in Ready](https://badge.waffle.io/luaposix/luaposix.png?label=ready&title=Ready)](https://waffle.io/luaposix/luaposix)
 
-luaposix is a POSIX binding, including curses, for [Lua] 5.1 and 5.2;
-like most libraries it simply binds to C APIs on the underlying
+luaposix is a POSIX binding, including curses, for [Lua] 5.1, 5.2 and
+5.3; like most libraries it simply binds to C APIs on the underlying
 system, so it won't work on non-POSIX systems. However, it does try
 to detect the level of POSIX conformance of the underlying system and
 bind only available APIs.