Auto merge of #42128 - kennytm:travis-folder, r=alexcrichton

Improve Travis CI log (travis_fold, colors)

Result looks like (https://travis-ci.org/rust-lang/rust/jobs/234614611):

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![1-fullpage](https://cloud.githubusercontent.com/assets/103023/26302841/2627c354-3f18-11e7-9299-52a2088639af.jpg)

</details>

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* Bring back colors on Travis, which was disabled since #39036. Append `--color=always` to cargo when running in CI environment.
* Removed `set -x` from the shell scripts. The `retry` function already prints which command it is running, adding `-x` just adds noise to the output. Interesting information can be manually `echo`ed.
* Support `travis_fold`/`travis_time`. Matching pairs of these allow Travis CI to collapse the output in between. This greatly cut down the unnecessary "successful" output one need to scroll through before finding the failed statement.
* Passed `--quiet` to all tests, so tests not failing will not occupy a line of log, reducing bloat in the report.

Also include some minor changes, like changing the `script` of `.travis.yml` to execute a single-line command, so the log won't write the extremely long multi-line
`The command "if [ "$ALLOW_PR" = "" ] && [ "$TRAVIS_BRANCH" != "auto" ]; then … " exited with 0` at the end.

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