Script syntax fix.

Author: Tom Barnes

wercker.yml

@@ -125,7 +125,7 @@ integration-test:
       # install kubectl
       # export K8S_CLIENT_VERSION="$(curl -s https://storage.googleapis.com/kubernetes-release/release/stable.txt)"   #latest version
       export K8S_CLIENT_VERSION="v1.10.5"
-      curl -LO https://storage.googleapis.com/kubernetes-release/release/$(K8S_CLIENT_VERSION)/bin/linux/amd64/kubectl
+      curl -LO https://storage.googleapis.com/kubernetes-release/release/${K8S_CLIENT_VERSION}/bin/linux/amd64/kubectl
 
       chmod +x ./kubectl
       mv ./kubectl /usr/local/bin/kubectl