Switch to https://build.servo.org

[Darkspirit]

@jdm I don't want to just ping you, so I make this pull request. This should help with https://github.com/servo/saltfs/issues/423#issuecomment-429399125, but is not tested with Vagrant.

• A next step would be to change it to a permanent http-to-https redirection and to activate HSTS.
• Current TLS config is bad, therefore only modern ciphersuites should be allowed.
• http://build.servo.org:54856/travis might already not exist anymore?
• Please feel free to edit this pull request.

(Background: servo.org enforces HSTS since bug 1482947 and is ready to be HSTS-preloaded.)

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This change is 

[Darkspirit]

@jdm There is a long list of travis webhooks trying to connect to http://build.servo.org:54856/travis which is already 404 Not Found when accessing with curl. Since #454 Homu is checking build statuses directly at GitHub https://github.com/servo/saltfs/blob/5dcb05a3d75430f3f8a2d78c4485bf0c2494059c/homu/files/cfg.toml#L268-L271 and doesn't rely on Travis webhook notifications, if I'm not wrong: https://github.com/servo/saltfs/blob/5dcb05a3d75430f3f8a2d78c4485bf0c2494059c/homu/files/cfg.toml#L100

Shall I create PRs removing each travis webhook? Or do you want to ignore them?

[jdm]

Removing those unneeded webhook notifications would be useful.

[Darkspirit]

I'll split this to multiple PRs as well.