Fix for GitHubResolver to use https instead of http for tarball archives

As noted in Issue 698, I was no longer able to use bower on our corporate network via a proxy. After much assistance and additional troubleshooting in both `request` and `bower`, I managed to isolate the issue.

`request` properly tunnels GitHub in bower, npm, etc. due to proper proxy config in .gitconfig and enviroment variables. However, it choked on http:// tarball URLs and would not resolve. Testing with a quick `curl` from behind this proxy, yeilded a 301 message which would never resolve within bower. 

Changing the URL to GitHub prefered 'https://' resolves properly, with or without proxy.