travis: make sure luarocks paths are set before installing EXTRA_ROCKS.

* travis.yml.in (script): Move @EXTRA_ROCKS@ after path setting.
* m4/slingshot.m4 (SS_CONFIG_TRAVIS): Remove SPECL_FALSE.
* travis.yml.in (script): Likewise.

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