travis: try moonscript gvvaughan manifest first, for Lua 5.3 rocks.

* m4/slingshot.m4 (SS_CONFIG_TRAVIS): AC_SUBST arguments to this
macro as EXTRA_ROCKS without munging.
* travis.yml.in (install): Install EXTRA_ROCKS using moonscript
gvvaughan manifest first, for Lua 5.3 compatible rocks not yet
available in the moonscript root manifest.
* .travis.yml: Regenerate.

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