astr: alien api cleanup.

It turns out that we can write code that references nonexisting
alien symbols, provided it isn't executed.
* lib/zile/astr.lua (have_memrchr): Just try to types method
to detect failure.
(rfind): Move out of loadstring.
Reported by Reuben Thomas

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
travis: remove workarounds for unreleased rocks.

* travis.yml.in (LDOC_ROCKSPEC, SPECL_ROCKSPEC): Remove.
* .travis.yml: Regenerate. Also removes STDLIB_ROCKSPEC pre-
release installation.

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