bootstrap: add --skip-rock-checks option.

* build-aux/bootstrap.slingshot (slingshot_options_prep): Add
default value and usage text for --skip-rock-checks option.
(slingshot_parse_options): Parse --skip-rock-checks option.
(slingshot_check_rockspecs): Early return if --skip-rock-checks
was passed.
* bootstrap: Regenerate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
travis: use a single luarocks tree.

Now that LDoc and LuaDoc work correctly on Lua 5.2, no need to
jump through hoops to keep a separate 5.1 rockstree for them.
* m4/slingshot.m4 (GENDOC, GENDOC_FALSE): Remove.
* travis.yml.in: Some extra comments and spacing for better
readability.
(global): Remove GENDOC export.
(install): Remove @GENDOC_FALSE@ section.
(script): Use new --skip-rock-checks when building a Makefile
for luarocks-config.lua generation.
* .travis.yml: Regenerate.

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