Auto merge of #3565 - integer32llc:fix-gh-pages, r=alexcrichton

Fix doc publishing: Remove $USER from the --user argument from pip install

[This looks to be the cause of why docs aren't publishing to gh-pages](https://travis-ci.org/rust-lang/cargo/jobs/193507734#L1059). I was able to publish the docs to [my repo's gh-pages](https://integer32llc.github.io/cargo/) with this change (and setting my own secure gh-token).

It looks like this problem got introduced [in this commit](https://github.com/rust-lang/cargo/commit/dc15ca5d202d6d10f6be45f388759a902723de6c). I don't really know python/pip, but `$USER` wasn't there before and [Travis' docs just say `--user`](https://docs.travis-ci.com/user/languages/python/#pip).