Shadow plugin version 1.0.1 is using an internal Gradle api class: org.gradle.api.internal.file.copy.CopySpecInternal in the class com.github.jengelman.gradle.plugins.shadow.internal.DependencyFilter.
This internal Gradle class is removed from Gradle 2.0 which was release at July 1st 2014, making shadow unusable with Gradle 2.0.