specl: make ansicolors an optional dependency.

* lib/specl.in: If ansicolors is not loadable, always disable
opts.color.
* lib/specl/color.lua: Don't raise an error when ansicolors is
not available.
* rockspec.conf (dependencies): Remove ansicolor.
* NEWS: Update.

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