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/PuerkitoBio/purell/.travis.yml

@@ -1,7 +1,12 @@
 language: go
 
 go:
-    - 1.4
-    - 1.5
-    - 1.6
+    - 1.4.x
+    - 1.5.x
+    - 1.6.x
+    - 1.7.x
+    - 1.8.x
+    - 1.9.x
+    - "1.10.x"
+    - "1.11.x"
     - tip