doc: update luarocks install instructions.

* README.md (Installation): Update to reference moonscript rocks
repository, which has better uptime than the default luarocks
domain used by released LuaRocks.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
maint: upgrade slingshot to work even when luarocks.org is down.

* slingshot: Upgrade to latest upstream.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
travis: use luarocks beta with rocks.moonscript.org.

* .travis.yml: Regenerate with latest slingshot.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
posix: support IPC_NOWAIT constant.

Closes #140.
* ext/posix/posix.c (luaopen_posix_c): Add IPC_NOWAIT constant.
Reported by Bernd Eggink.

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