.travis.yml: make use of travis haxe support
.travis.yml: make use of travis haxe support

Now the tests are run against Haxe 3.2.0 and 3.1.3.
Gama11/openfl,Gama11/openfl,Gama11/openfl,NenadBojkovski/openfl,openfl/openfl,NenadBojkovski/openfl,openfl/openfl,Gama11/openfl,openfl/openfl