use correct gem name and homepage

I think the correct gem name should be galetahub-simple_captcha. Changing its name to "simple_captcha" make it inconsistent to install from different source

I also found it confusing when clicking "Homepage" on rubygems.org redirects me to another github repository (http://github.com/izzm/simple-captcha)

We may need more inputs from @izzm for the reason of changing gem name.