Fix homepage URL in gemspec

Hi!

I noticed the homepage URL at <https://rubygems.org/gems/httpi> points to <http://github.com/savonrb/#{s.name}>, which I guess is not intentional! Using " quotes instead of ' in the gemspec means the variable gets expanded properly.

Thanks for httpi!
Sam
Hi @ssssam! Thanks for the PR. This "r" is breaking gemspec. Do you mind if i change this on master and we close this PR?