configury: bump SPECL_MIN to 8.

* configure.ac (SPECL_MIN): Bump to 8, because we're using the
Specl 8 hell.spawn APIs now.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
configury: travis builds with 5.2 require luafilesystem rock.

* configure.ac (SS_CONFIG_TRAVIS): Add luafilesystem to extra
rocks list.

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