Gradle DSL Method not found: testCompile()

Hi I am working on a project and have reached a stage where it would be unwise to continue without unit testing. I thought unit testing in Android Studio would be straight forward but I can't run my tests.

Here are my dependencies. Everything works fine until I add the last line of code. I then receive the error message that is in the title.

How do I fix this and run my tests?

Here was the tutorial I was following: http://ift.tt/1KkPIk8