travis: fix Makefile snippets from slingshot to work standalone.

* Makefile (set_LUA_BINDIR): Use literal `sed`.
(luarocks-config.lua): Unquote $(abs_srcdir) for the shell.
(LUA_PATH): Let environment LUA_PATH take precedence.
* .travis.yml (LUAROCKS_BASE): Revert to 2.1.1, which configures
with the provided arguments.
(install): Add stdlib luarock.
maint: explicit `tostring` in string.format for 5.1 compat.

* lib/sexpr/primitive.lua (defmacro, lambda): Use explicit
`tostring` in `string.format` parameters.
* specs/parser_spec.yaml: Likewise.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
specl: use an unreleased specl with tail-calling objcmp.

Released specl overflows luajit stack when comparing 1000 nested
objects. Pull in the fixed version until Specl 11 is released.
* build-aux/specl: New file, with tail-calling objcmp.
* Makefile (SPECL): Use it.
* lib/sexpr/lisp.lua (ComparableAtom): New root object with fast
comparison metamethods.
(Number, String): Use it.

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