maint: update rockspec template for latest ax-lua.m4.

* lyaml-rockspec.lua (build.build_command): Pass $(LUA) as a
configure precious variable so that latest ax-lua.m4 uses it as
passed instead of running the hardcoded search for a lua binary.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
configury: user overridable luarocks vars in luarocks-config.lua.

* GNUmakefile (luarocks-config.lua): In order to work correctly
with a given lua intepreter for a particular luarocks tree, make
sure to set the appropriate luarocks variables when writing a
luarocks-config.lua. Also, take care to honour environment
values for those variables at make-time, because the user probably
knows better how their environment is set up than the generic
make rules can!

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
maint: use a local luarocks tree for Travis CI.

* .travis.yml (install): Don't forget the various foo-dev packages
needed to get lua header files.
(after_script): Remove. There are no tests!
(script): Build and use a luarocks-config to use a temporary
local luarocks tree tailored to the selected LUA interpreter.

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