integrations: use the snapcore/snapcraft docker image in travis

[filibtester]

Thanks for helping us make a better Snapcraft!

Checklist:

• Have you signed the contributor licence agreement?
  https://www.ubuntu.com/legal/contributors

• Is there a reported a bug for the problem you are fixing?
  https://bugs.launchpad.net/snapcraft/
  If there is none, please report it. Once you have the BUG_NUMBER,
  replace it here:

LP: #BUG_NUMBER

• Have you read our contribution guide?
  CONTRIBUTING.md

[come-maiz]

@filibtester thanks for the fix!
But you haven't signed the contributors agreement. Take a look at the first point of the checklist up there ^

[come-maiz]

For other reviewers, this change is interesting. Take a look that @filibtester moved the file, because it wasn't being run by ./runtests.sh unit.
So this test got stale, and started failing without us noticing.

[cprov]

@filibtester nice catch! Thank you.

[come-maiz]

This is missing one more +1. @kalikiana or @kyrofa, please take a look.

[kyrofa]

This is missing one more +1. @kalikiana or @kyrofa, please take a look.

Haha, as far as I can see, it doesn't have a +1 from you either! This looks good to me, but I'd appreciate a quick look from @cprov as well.

[come-maiz]

Sorry, I forgot my approval.

[cprov]

The change looks good and will certainly bump usage of the snapcraft docker image.

Just a small remark, this change doesn't offer any alternative for updating existing integrations, snapcraft enable-ci travis --refresh will continue to only update store credentials. The .travis.yml changes are simple enough this time and probably don't justify investing time for assisted updates.

[come-maiz]

@cprov That's a good point, about the existing integrations out there. However, keeping the old image is safe, as it takes care of upgrading snapcraft and will result in the same snap. And the work required to migrate those old projects would be complex, not required IMO. I'm going to merge this one.