docs(ci): Remove Travis example

When the guide was written, it only included Travis CI.  For while, it
was *the* CI service people used.  However, since git hosting services
have been integrating CI support and with Travis CI's new owners, market
share has been going down.  One estimate I found said that Travis CI's
market share is 0.94%.  If we had a compelling reason to include it,
independent of that, I would.  However, including it and making it first
in the list is a distraction.  There are CI services in more common use
(e.g. CircleCI, Jenkins) to include at this point over Travis.

Author: epage

src/doc/src/guide/continuous-integration.md

@@ -1,26 +1,5 @@
 ## Continuous Integration
 
-### Travis CI
-
-To test your [package][def-package] on Travis CI, here is a sample
-`.travis.yml` file:
-
-```yaml
-language: rust
-rust:
-  - stable
-  - beta
-  - nightly
-matrix:
-  allow_failures:
-    - rust: nightly
-```
-
-This will test all three release channels, but any breakage in nightly
-will not fail your overall build. Please see the [Travis CI Rust
-documentation](https://docs.travis-ci.com/user/languages/rust/) for more
-information.
-
 ### GitHub Actions
 
 To test your package on GitHub Actions, here is a sample `.github/workflows/ci.yml` file: