Announce the source of env vars

Some time ago I added support of env vars from repository settings to travis-build, but it lacked a distinction between setting env vars from config file and from settings. This PR contains an echo statement announcing each env var group, for example:

```
Setting environment variables from repository settings
export FOO=bar
```

Nope, because travis vars already return `nil` from `to_s`, this code is the same as the previous version.