travis: restore library links for system libyaml.

Recent Travis CI VMs are missing libraries for libyaml in the
standard compiler search path.  Put them back in that case.
* travis.yml.in (install): If libyaml.so is not in /usr/lib,
search for suitable libraries and link them into place.

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