Fixed syntax error

```
[error] /localhome/jenkins/c/workspace/scalameter-wip-nightly/src/main/scala/org/scalameter/dsl.scala:15: not enough arguments for constructor DynamicVariable: (init: () => Any)scala.util.DynamicVariable[() => Any].
[error] Unspecified value parameter init.
[error]   private[scalameter] val testbody = new DynamicVariable[() => Any]{_ =>
[error]                                          ^
[error] one error found
[error] (compile:compile) Compilation failed
```
https://scala-webapps.epfl.ch/jenkins/view/All/job/scalameter-wip-nightly/1/console