Replace Google Code URLs with corresponding GitHub URLs

The `guava-parent` Maven POM still contains URLs pointing to Google Code for issues and SCM. This changesets updates the respective URLs to their counterparts on GitHub.
GitHub does provide read/write access using HTTPS and it's their recommended way of access (see https://help.github.com/articles/which-remote-url-should-i-use/#cloning-with-https-recommended).

Personally I also like access via SSH better and I can change the `<developerConnection>` URL if you prefer that over HTTPS.