Using the new 'plugins' syntax to apply core plugin wrapper.
Brought back Travis CI config.
Added BINTRAY properties as Travis CI env parameters in order to pass the build.
Removed env properties as they can be configured as repository settings.
Brought back build badge.
Declared bintray plugin explicitly.

Added TestNG test configuration.

Added 'gradle-plugin' attribute to pkg settings for Bintray publication.

Optionally add empty BINTRAY_USER and BINTRAY_API_KEY properties to 'ext' if they are not present during build (Travis CI).
The integration test plugin now applies the core 'java' plugin by default so that sourceSets are always available. Added some unit tests for the integration-test plugin also.
Updated documentation on how to use the integration-test plugin via Gradle plugin portal.
Changed CI build task to 'build'.