Improve travis automation

[depili]

https://github.com/veandco/go-sdl2/blob/master/.travis.yml#L18 should be augmented to building all of the examples, currently the go build command does nothing at all and will always exit with status 0.

[depili]

Actually just changing the go build to "go build github.com/veandco/go-sdl2/{sdl,img,mix,ttf}" should also improve things quite nicely. That would have caught #386

[veeableful]

Hi @depili, nice suggestion! I have changed the Travis configuration file accordingly. The CI should build the other packages now.