Add isTravisCI

Checks if the current environment is Travis CI.

snippets/isTravisCI.md

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+### isTravisCI
+
+Checks if the current environment is [Travis CI](https://travis-ci.org/).
+
+Checks if the current environment has the `TRAVIS` and `CI` environment variables ([reference](https://docs.travis-ci.com/user/environment-variables/#Default-Environment-Variables)).
+
+```js
+const isTravisCI = ()) => 'TRAVIS' in process.env && 'CI' in process.env;
+```
+
+```js
+isTravisCI(); // true (if code is running on Travis CI)
+```