eval: require stdlib explicitly.

* lib/zemacs/eval.lua, lib/zz/eval.lua: Don't rely on a global
`std` symbol from bin/zemacs or bin/zz, because we need to run
eval functions separately.  Add explicit `require "std"`.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
specl: clean-up now that Specl 12 is in the wild.

* bootstrap.conf (buildreq): Cite release url for Specl 12
rockspec.
* .travis.yml: Regenerate.

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