Update our dependency on K8s libs to 1.15.3

With a minimum K8s version of 1.14 (starting in 0.10), 1.15.3 puts us in the center of the +/-1 version window of support.

/hold

This depends on a number of upstream PRs to land, including in the K8s incubator.

Gopkg.toml

@@ -28,11 +28,17 @@ required = [
 
 [[override]]
   name = "knative.dev/pkg"
-  branch = "master"
+  #TODO(mattmoor): DO NOT SUBMIT
+  source = "github.com/mattmoor/pkg-1"
+  revision = "797805c33315dc62c1609a36202c37cfcdaffbb3"
+  # branch = "master"
 
 [[constraint]]
   name = "knative.dev/caching"
-  branch = "master"
+  #TODO(mattmoor): DO NOT SUBMIT
+  source = "github.com/mattmoor/caching"
+  revision = "9fd14e2ad888e756c2589bd5cf17d624134f4042"
+  # branch = "master"
 
 [[override]]
   name = "github.com/google/mako"
@@ -53,23 +59,41 @@ required = [
 
 [[override]]
   name = "k8s.io/api"
-  version = "kubernetes-1.12.9"
+  version = "kubernetes-1.15.3"
 
 [[override]]
   name = "k8s.io/apimachinery"
-  version = "kubernetes-1.12.9"
+  version = "kubernetes-1.15.3"
 
 [[override]]
   name = "k8s.io/code-generator"
-  version = "kubernetes-1.12.9"
+  version = "kubernetes-1.15.3"
 
 [[override]]
   name = "k8s.io/apiextensions-apiserver"
-  version = "kubernetes-1.12.9"
+  version = "kubernetes-1.15.3"
 
 [[override]]
   name = "k8s.io/client-go"
-  version = "kubernetes-1.12.9"
+  version = "kubernetes-1.15.3"
+
+[[override]]
+  name = "k8s.io/apiserver"
+  version = "kubernetes-1.15.3"
+
+[[override]]
+  name = "k8s.io/metrics"
+  version = "kubernetes-1.15.3"
+
+[[override]]
+  name = "k8s.io/kube-openapi"
+  # This is the version at which k8s.io/apiserver depends on this at its 1.15.3 tag.
+  revision = "b3a7cee44a305be0a69e1b9ac03018307287e1b0"
+
+[[override]]
+  name = "sigs.k8s.io/structured-merge-diff"
+  # This is the version at which k8s.io/apiserver depends on this at its 1.15.3 tag.
+  revision = "e85c7b244fd2cc57bb829d73a061f93a441e63ce"
 
 [[override]]
   name = "github.com/rogpeppe/go-internal"
@@ -82,15 +106,10 @@ required = [
 # Added for the custom-metrics-apiserver specifically
 [[override]]
   name = "github.com/kubernetes-incubator/custom-metrics-apiserver"
-  revision = "b7016fc85e1c960758aef23e1d456a66997d1e26"
-
-[[override]]
-  name = "k8s.io/apiserver"
-  version = "kubernetes-1.12.9"
-
-[[override]]
-  name = "k8s.io/metrics"
-  version = "kubernetes-1.12.9"
+  #TODO(mattmoor): DO NOT SUBMIT
+  # Pulls in https://github.com/kubernetes-incubator/custom-metrics-apiserver/pull/55
+  source = "github.com/CharlyF/custom-metrics-apiserver"
+  revision = "4e0c32d9b586de84fb8d9399134e8e5b912a74eb"
 
 [[override]]
   name = "bitbucket.org/ww/goautoneg"