Fix binding when determining version from gemspec

Some gems don't use a namespaced `VERSION` constant.  When publishing these gems, gem_publisher would push a tag with it's own version instead of the gem's version.  This is because the gemspec was evaled within the
`GemPublisher` module, and therefore the `GemPublisher::VERSION` constant was picked up.

Admittedly, these gems aren't well written, but picking up gem_publisher's version is definitely an error.

eval'ing with the `TOPLEVEL_BINDING` solves this.