Auto merge of #44399 - alexcrichton:tmp-travis-downgrade, r=alexcrichton
Auto merge of #44399 - alexcrichton:tmp-travis-downgrade, r=alexcrichton

travis: Downgrade to previous images temporarily

Travis is in the process of [rolling out an update][update] but looks like our
tests are breaking, let's temporarily roll back to get the queue moving again.

[update]: https://blog.travis-ci.com/2017-08-29-trusty-image-updates

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