Update travis.yml with new k8s version and updated kind version

.travis.yml

@@ -5,14 +5,15 @@ language: go
 go:
   - 1.13.x
 env:
+  - K8S_VERSION=v1.18.2
   - K8S_VERSION=v1.17.0
   - K8S_VERSION=v1.16.4
   - K8S_VERSION=v1.15.7
 services:
   - docker
 before_script:
   - curl -Lo kubectl https://storage.googleapis.com/kubernetes-release/release/${K8S_VERSION}/bin/linux/amd64/kubectl && chmod +x kubectl && sudo mv kubectl /usr/local/bin/
-  - wget https://github.com/kubernetes-sigs/kind/releases/download/v0.7.0/kind-linux-amd64
+  - wget https://github.com/kubernetes-sigs/kind/releases/download/v0.8.1/kind-linux-amd64
   - chmod +x kind-linux-amd64
   - mv kind-linux-amd64 kind
   - export PATH=$PATH:$PWD