The build fails only in release, debug builds fine.
Android studio 1.4, Gradle 2.8 RC1, Gradle plugin 1.4.0 BETA 4
FAILURE: Build failed with an exception. * Exception is: org.gradle.api.GradleException: Could not determine the dependencies of task ':app:crashlyticsStoreDeobsRelease'. ... Error:org.gradle.api.UnknownTaskException: Task with path 'dexRelease' not found in project ':app'.
This is due to the changes made to Gradle 1.4.0.beta2
You need to update the fabric Gradle plugin to a version > 1.20.0:
classpath 'io.fabric.tools:gradle:1.20.1' should do it