travis: be explicit about which LuaRocks tree is being used.

Older LDoc, and all Luadoc release only work with Lua 5.1, so on
Travis we have a system luarocks configured with Lua 5.1 to hold
those, but a separate LuaRocks tree managed by the latest LuaRocks
release for rock dependencies.
* travis.yml.in: Specify the system luarocks tree for bootstrap,
so that it will find LuaDoc correctly.

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