Update to use go modules (#671)

This includes updates to CI config, vendor files, etc.
I've hard-coded the k8s depedency at 1.13.4 to keep it inline with what
we currently have and to make sure a another run of `go mod tidy`
doesn't accidentally update it to an unexpected version.

Thanks to hectorj2f for carrying this along.

vendor/go.opencensus.io/internal/traceinternals.go

@@ -22,6 +22,7 @@ import (
 // TODO(#412): remove this
 var Trace interface{}
 
+// LocalSpanStoreEnabled true if the local span store is enabled.
 var LocalSpanStoreEnabled bool
 
 // BucketConfiguration stores the number of samples to store for span buckets