Rollup merge of #45321 - alexcrichton:xcodeup, r=aidanhs
Rollup merge of #45321 - alexcrichton:xcodeup, r=aidanhs

ci: Update Travis OSX builders

Looks like Travis [has announced][blog] that our current `xcode8.2` image is
being deprecated and the recommended Xcode 7 image is `xcode7.3`. This updates
us to these ahead of time to make sure we can shake out any bugs, if any.

[blog]: https://blog.travis-ci.com/2017-10-16-a-new-default-os-x-image-is-coming

graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust