specl: don't inject a Lua return keyword into a YAML spec.

Assume specs are written in YAML, and only fallback to legacy
Lua format if that fails.
* specl/specl.in (slurp): Try yaml.load first, and loadstring
with an injected return keyword only if yaml.load failed.
Reported by François Perrad.

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