Add 'depends_on' to CommandStep (#7)

We have a requirement for certain steps to be dependent on each other,
without relying on static waits.

buildSrc/src/main/groovy/com/widen/plugins/buildkite/BuildkitePipeline.groovy

@@ -207,6 +207,13 @@ class BuildkitePipeline implements ConfigurableEnvironment {
             model.key = key
         }
 
+        /**
+         * Make this step depend on the completion of another step
+         */
+        void dependsOn(String dependsOn) {
+            model.depends_on = dependsOn
+        }
+
         /**
          * Add a Buildkite plugin to this step.
          *