[Travis CI] Missing new line at the end of deploy key

Hi 👋,

when setting up an automatic deployment using Travis CI, I've followed commands from documentation, but this command caused the Travis CI job to fail by asking me for `passphrase`

```
travis env set -- DEPLOY_KEY "$(cat deploy_key)"
```

The cause was in missing new line at the end, so I've used slightly updated command, which made the job to pass

```
travis env set -- DEPLOY_KEY "$(cat deploy_key)"$'\n'
```

I'm not sure if I did something wrong, or the docs should be updated, if this affects more people?