[TravisCI] Add a build for the minimum sdk requirements in the build matrix

[vicb]

Currently we run test for both the stable and the bleeding edge versions of the SDK on Travis.

I think it would make sense to also test with the version specified in the pubspec as I expect most devs to only test with the BE (or at least latest stable) version.

This is something I could implement if you think it would be valuable ?

see #878 (comment)

[jbdeboer]

Is that field in the pubspec actually used for anything?

There is no support from the Dart team for anything older than stable, so we can't support old versions either. We should flip this problem around and always keep the environment: sdk value at the current stable release.

[vicb]

I would assume it would at least print an error message if the SDK version is lower than what's require. I'll update the pubspec.yaml then.

[vicb]

Is that field in the pubspec actually used for anything?

The answer to this question can be found in the linked PR: it fails when the req is not fulfilled.