Deploy to github-pages with travis

Travis support is now working. [Project is being correctly built](https://travis-ci.com/GabLeRoux/unity3d-ci-example/builds/98106936) and it's only missing a job in `.travis.yml` to automatically push to `gh-pages` with the webgl artifact to deploy to [github-pages](https://pages.github.com/)

Here's the related documentation to accomplish this:
https://docs.travis-ci.com/user/deployment/pages/

We will need [build stages](https://docs.travis-ci.com/user/build-stages/)

The deploy job will need to run after the webgl build success and deploy the artifact stored in `./Builds/WebGL/`.

## Draft

```yaml
- stage: deploy
if: type = push AND branch = master OR branch = feature/gh-pages # temporary branch to test the deployment
script: skip
deploy:
- provider: pages
skip_cleanup: true
github_token: $GH_TOKEN # Set in the settings page of your repository, as a secure variable
keep_history: true
local_dir: ./Builds/WebGL/
```

Start with .travis.yml and the Travis CI documentation on deployment to GitHub Pages and build stages. Verify that the deploy stage runs after the WebGL build and uses the artifact in ./Builds/WebGL/. Done means a push to the intended branch successfully publishes that artifact to gh-pages.

Written by the indexing model from the issue text.