Be more specific about what to include from bin

Hey @glenn-murray-bse! I think this will resolve the issue that @PragTob
was having with your PR over on #919. Can you confirm that this
will still properly install and run on Windows where it wouldn't before?

If so, then you should be able to merge my changes into your branch here,
and #919 will pick up my additional commit as well.

Thanks so much for hopping onto the fix for that! It takes me a lot of time
and effort to get my super-slow old Windows machine up and running
for these type of issues, so getting in there on it was a huge help. :sparkles: 

**What my change here is about**
When we started including the executables in shoes.gemspec, the
existence of shoes.bat (which is a symlink on *nix style systems)
started causing problems when we try to build the gem.

As it turns out, we don't actually need that file in bin as part of the
gem's packaging--it is copied from ext/install during installation
anyway, and only linked into bin as a convenience for running from
source on Windows.

With this change, we can package again on *nix systems and hopefully
continue to work properly with the changed paths on Windows.