maint: Ubuntu Precise ships too-old luarocks.

* .travis.yml (env.matrix): Use correct binary name for luajit.
(env.global): Set LUAROCKS_BASE to current luarocks release.
(install): Don't apt-get too-old luarocks from distro.
(script): Wget and build a local luarocks from latest release.

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