HXCPP_SILENT vs Travis

`HXCPP_SILENT` is a great way to reduce output verbosity in CI environments such as Travis (where you can actually only view the log as raw text after it reaches a certain length). However, it doesn't play nice with Travis specifically because it terminates builds after 10 minutes of not receiving any output:

https://travis-ci.org/Gama11/flixel/jobs/384326889#L866

It's possible to work around this, but it's not pretty (from https://github.com/travis-ci/travis-ci/issues/4190#issuecomment-353342526):

```yml
script:
- while sleep 540; do echo "=====[ $SECONDS seconds still running ]====="; done &
- haxe --cwd tests --run RunTravis $TARGET $OPENFL
- kill %1
```

There's several solutions I could think of:

- just let the HXCPP build tool _always_ print something after every n minutes
- do that, but only when it detects it's running on Travis (check if the `TRAVIS` env var is defined)
- add some `HXCPP_KEEP_ALIVE=`-define for enabling the print behavior

Perhaps a combination if option 2 + 3 would be best?