maint: use proven Travis CI build recipes from lyaml.

* .travis.yml: Overhaul according to lyaml recipes.
* GNUmakefile (luarocks-config.lua): In order to work correctly
with a given lua interpreter for a particular luarcoks tree, make
sure to set the appropriate luarocks variables when writing a
luarcoks-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!
* README.md: Add travis build status badge markup.
* build-aux/mkrockspecs.lua: Use the user selected luarocks
invocation to execute luarocks lint.
* specl-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>