cleanup the gemspec - Githubissues

Highlights

use metadata to specify the homepage and changelog URI
- the current links are incorrect (homepage github.com/openzipkin/zipkin-tracer, source code openzipkin/zipkin-ruby.git)
- rubygems doesn't support setting links elsewhere anymore, from this post:
  
  As of now, if you need to update any of the sidebar links previously editable from the UI, you would have to set a few URI attributes in spec.metadata
- source code link not needed as it'd be the same as the homepage
removed required_rubygems_version, we're requiring ruby 2.3 and it ships with rubygems 2.5.1
updated the contributors list (added Yohei, mdsol.com → medidata.com)

I believe bumping the version is the only way to have those links fixed on rubygems.org, let me know if otherwise.

@adriancole @cabbott @jcarres-mdsol @ykitamura-mdsol @ssteeg-mdsol @piao-mdsol