Handle 403 gracefully

[petems]

It's quite easy to get rate-limited whilst using the gem I've found out.

Would be cool to add a way of handling 403's gracefully 👍

When using the

{
  "message": "API Rate Limit Exceeded for <ip address>"
}

Current behaviour:

from /opt/boxen/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/open-uri.rb:775:in `buffer_open'
    from /opt/boxen/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/open-uri.rb:203:in `block in open_loop'
    from /opt/boxen/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/open-uri.rb:201:in `catch'
    from /opt/boxen/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/open-uri.rb:201:in `open_loop'
    from /opt/boxen/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/open-uri.rb:146:in `open_uri'
    from /opt/boxen/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/open-uri.rb:677:in `open'
    from /opt/boxen/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/open-uri.rb:33:in `open'
    from /opt/boxen/rbenv/versions/1.9.3-p392/lib/ruby/gems/1.9.1/gems/contributions-0.2.0/lib/contributions/github_api.rb:40:in `repos'
    from /opt/boxen/rbenv/versions/1.9.3-p392/lib/ruby/gems/1.9.1/gems/contributions-0.2.0/lib/contributions/github_api.rb:11:in `forks'
    from /opt/boxen/rbenv/versions/1.9.3-p392/lib/ruby/gems/1.9.1/gems/contributions-0.2.0/lib/contributions/contributions.rb:130:in `setup_repositories'
    from /opt/boxen/rbenv/versions/1.9.3-p392/lib/ruby/gems/1.9.1/gems/contributions-0.2.0/lib/contributions/contributions.rb:11:in `initialize'
    from testscript.rb:4:in `new'
    from testscript.rb:4:in `<main>'

Possible new behaviour:

Warning: You have exceeded your API limit, please wait