AdaNet release 0.9.0.

PiperOrigin-RevId: 320458134

Release AdaNet v0.8.0.

Also update Travis to 2.0.0 stable, and update documentation about TF 2 support.

PiperOrigin-RevId: 272451019

Release AdaNet v0.7.0.

PiperOrigin-RevId: 255251259

Fix Bazel build error introduced by 9e9c396.

PiperOrigin-RevId: 245824058

Release version v0.6.1.

PiperOrigin-RevId: 240836011

Release AdaNet v0.6.0.

PiperOrigin-RevId: 240649904

Release v0.5.0.

Add missing documentation for TrainOpSpec.

PiperOrigin-RevId: 225897121

Release AdaNet v0.4.0.

PiperOrigin-RevId: 223386399

Release AdaNet v0.3.0.

PiperOrigin-RevId: 220499302

Fix typo in README.

PiperOrigin-RevId: 219691757