use istio client-go library instead of knative (#1661)

use istio client-go library instead of knative
bump kubernetes dependency version
change code coverage to codecov

vendor/github.com/gregjones/httpcache/.travis.yml

@@ -1,18 +0,0 @@
-sudo: false
-language: go
-matrix:
-  allow_failures:
-    - go: master
-  fast_finish: true
-  include:
-    - go: 1.10.x
-    - go: 1.11.x
-      env: GOFMT=1
-    - go: master
-install:
-  - # Do nothing. This is needed to prevent default install action "go get -t -v ./..." from happening here (we want it to happen inside script step).
-script:
-  - go get -t -v ./...
-  - if test -n "${GOFMT}"; then gofmt -w -s . && git diff --exit-code; fi
-  - go tool vet .
-  - go test -v -race ./...