travis: install luarocks to work properly with compiled rocks.
travis: install luarocks to work properly with compiled rocks.

* travis.yml.in (matrix): Add LUA_SUFFIX setting per Lua.
(install): configure luarocks with matching version, suffix and
include settings.

Signed-off-by: Gary V. Vaughan <f9023000f29773649f3850298becb9544b5fd6a9@gnu.org>

gvvaughan/slingshot