maint: use local luarocks paths for make rockspecs.

* .travis.yml (env): Move LUA intepreters into global subkey.
(env.matrix): Set LUAROCKS_CONFIG once for everything here.
(install): Remove redundant lua interpreter installs.
(script): Set paths for local luarocks tree before calling `make
rockspecs`, otherwise it can't find the stdlib rock we just
installed.

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