See #34 :
With version 2.1.1+4-9d76569a and sbt +releaseEarly I get:
java.lang.RuntimeException: Missing bintray credentials. Either create a credentials file with the bintrayChangeCredentials task, set the BINTRAY_USER and BINTRAY_PASS environment variables or pass bintray.user and bintray.pass properties to sbt.
even though my build.sbt specifies to publish using sonatype.
The full travis log is also available.
PR #37 provides a potential mitigation.