Make api gemspec uniform to other ones
Make api gemspec uniform to other ones

This commit just changes the inner block variable `gem` to `s`,
to reflect what we do in other gemspecs.

It also reorder items so it's easy to find gemspecs pieces where
we have them in the gemspec of the other components.

pervino/solidus,pervino/solidus,pervino/solidus,pervino/solidus