How to make task depend on another in sbt 0.12?

I'm using SBT 0.12.0.

I've two tasks in my project/Build.scala - helloTask and u2 defined as follows:

val hello = TaskKey[Unit]("hello", "Prints 'Hello World'")

val helloTask = hello := {
  println("Hello World")
}

val u2Task = TaskKey[Unit]("u2") := { println("u2") }

How to make u2 task depend on hellotask? I used <<= following the sample as described in Tasks (in the original version of the question it was https://github.com/harrah/xsbt/wiki/Tasks, but the doc has since moved and changed).

u2Task <<= u2Task dependsOn helloTask

But I got reassignment to val error. Apparently, I cannot get anything with <<= to work. What am I doing wrong?