You must define BINTRAY_USER and BINTRAY_API_KEY as environment variable.
Bintray API key can be found under Edit Profile -> API Key.
Be sure that build.gradle and Readme are both updated and contain new, to be released version.
./gradlew clean build javadocs assemble
./gradlew bintrayUpload
Manually go to Bintray page, check that all 8 files are there and publish them if everything is OK.