Rebuilding a "Fixed" or "Broken" build resends the email

[jiri]

When rust-ci rebuilds a push that fixed a project, the "Fixed" or "Broken" notification email is resent to the owner, which can result in quite a lot of spammy emails.

Steps to reproduce:

• Use rust-ci
• Have a broken repo
• Push a fix
• Wait for rust-ci to rebuild after a nightly

Suggested fix:

Either making a new build (not rebuilding) or consulting with the Travis team on a better way to fix this.