Remove duplicate Gulp screenshots

Only need one!
Remove useless alt text

If we’re not going to provide useful alt text, spec/best practice is
to have none at all, since it’s used by screen readers, etc.
Add back in missing Chef screenshot

Looks like it was removed from the README by accident during the
v2 refactor.
Remove nonexistent Ruby screenshot

If it ever did exist, it’s been gone a while.
Merge pull request #43 from jeffbyrnes/cleanup-readme

Yet more README cleanup!