Activate gradle build scans for CI builds

[britter]

As discussed via twitter this PR will activate build scans for CI builds. For an example see this scan of a local build on my machine.

I had to add some configuration to build.gralde for this:

• The build scan plugin is applied first. Otherwise it will not collect all data for Gradle < 4.3.
• The terms of service are accepted via the corresponding configuration in build.gradle. Every build that is called with the --scan option will be published. @melix suggested to add a more sophisticated configuration, that will also publish all local builds if user accept it. For now I've only added the simple configuration.

Furthermore I've added the --scan option to all travis stages.

[kiview]

While I generally like feature (although I haven't looked into the scan result in much detail), I wonder if there is a way, to make the scan result public by default?

Right now, one has to activate the scan result by visiting the URL logged by TravisCI and providing an email. I think it would be much nicer, if they would be activated by default, but maybe I'm misreading the actual workflow (being mainly an on-demand one).

We could add the --scan flag to CirceCI as well.

[kiview]

Never mind my comment, seems this was for creating an user account.

[rnorth]

I think this looks fine to me, thank you!
As the scan only occurs on CI builds I can't see there being any privacy risk to contributors.