travis: build flatpak using branch from PR instead of next

[larchunix]

Currently, travis flatpak build always uses the tip of next branch. This means builds triggered by a pull request does not test the changes introduced by the PR since the code is not merged in next branch yet.

This PR aims to fix this.

[antenore]

Thanks @larchunix
The only issue is that the build takes around 20 minutes.

Sometimes it's just awful to wait, that's why we decided to build only next.

Well, I prefer to wait and be sure everything works.

[amtlib-dot-dll]

Thanks @larchunix
@antenore Probably we can utilize the cache and build stages features of Travis?

[antenore]

@amtlib-dot-dll I forgot to answer to you, sorry.

Well, yes! Do you know ho to do it? Otherwise I'll take a look when I'll be back from vacation and as soon as possible (i'm quite busy with my duties ;-) )

[amtlib-dot-dll]

Well me too, but as users are developers for free softwares, we are not alone, are we? 😉

[antenore]

I've enabled the travis cache trough #1619
We will see if it works with the next PR