kgio 2.11.1 - fix Ruby 2.5.0dev compatibility

This release fixes compatibility with Ruby 2.5.0 preview and
release candidates (and thus should be ready for 2.5.0 final)
on some platforms.  At least clang 5.0.0 on OpenBSD is affected.

Thanks to Jeremy Evans for the fix.

There's also minor documentation updates since 2.11.0