maint: preliminary Lua 5.3.x compatibility.

* configure.ac (AX_PROG_LUA): Accept Lua 5.3 interpreters.
* src/merge-sections.in, src/mkrockspecs.in: Search for lua5.3 or
lua53 when re-executing with shell preamble.
* build-aux/merge-sections: Regenerate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
travis: support Lua 5.3 matrix.

* .travis/platform.sh, .travis/setup_lua.sh: New scripts to setup
lua and luarocks on travis, from luafilesystem project.
* travis.yml.in: Simplify accordingly.
(matrix): Add Lua 5.3.
* m4/slingshot.m4 (SS_CONFIG_TRAVIS): Generate EXTRA_ROCKS to
match simplified .travis.yml.
* .travis.yml: Regenerate.

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