Remove Travis configuration from flutter/flutter (#20288)

This removes the final traces of Travis and Appveyor from the Flutter tree.

I've updated the documentation and fixed a couple of places where scripts look for Travis, and eliminated the dart tools runningOnTravis function (which was unused anyhow).

There are places in the flutter script that used to look for the environment variable TRAVIS. We actually do want to continue to detect that we're running on Travis there, since in the plugins repo we still use Travis (for the moment). In any case, it's OK, because the CI environment variable is set on all of the CI bots (Cirrus, Travis, and Appveyor).

FastLane doesn't have a setup_cirrus equivalent to setup_travis, but it actually doesn't matter there either, since it doesn't do Travis-specific things, and it also looks for the CI environment variable.

CONTRIBUTING.md

@@ -1,7 +1,7 @@
 Contributing to Flutter
 =======================
 
-[![Build Status](https://travis-ci.org/flutter/flutter.svg)](https://travis-ci.org/flutter/flutter)
+[![Build Status](https://api.cirrus-ci.com/github/flutter/flutter.svg)](https://cirrus-ci.org/flutter/flutter)
 
 _See also: [Flutter's code of conduct](https://flutter.io/design-principles/#code-of-conduct)_
 
@@ -129,7 +129,7 @@ which provides flutter-specific extensions on top of [package:test](https://pub.
 option to start the test in a paused state and wait for connection from a debugger. This option lets you
 set breakpoints before the test runs.
 
-To run all the tests for the entire Flutter repository, the same way that Travis runs them, run `dart dev/bots/test.dart`.
+To run all the tests for the entire Flutter repository, the same way that Cirrus runs them, run `dart dev/bots/test.dart`.
 
 If you've built [your own flutter engine](#working-on-the-engine-and-the-framework-at-the-same-time), you
 can pass `--local-engine` to change what flutter shell `flutter test` uses. For example,
@@ -190,7 +190,7 @@ To send us a pull request:
 Please make sure all your checkins have detailed commit messages explaining the patch.
 
 Once you've gotten an LGTM from a project maintainer and once your PR has received
-the green light from all our automated testing (Travis, Appveyor, etc), and once
+the green light from all our automated testing (running on Cirrus, etc), and once
 the tree is green (see the [design principles](https://flutter.io/design-principles/)
 document for more details), submit your changes to the `master` branch using one of
 the following methods:
@@ -285,7 +285,6 @@ Build infrastructure
 
 We build and test Flutter on:
 
-- Travis ([details](.travis.yml))
-- AppVeyor ([details](appveyor.yml))
+- [Cirrus](https://cirrus-ci.com/) ([details](.cirrus.yml))
 - Chromebots (a.k.a. "recipes", [details](dev/bots/README.md))
 - Devicelab (a.k.a. "cocoon", [details](dev/devicelab/README.md))