vendor: update buildkit to v0.8

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/google.golang.org/appengine/.travis.yml

@@ -10,8 +10,6 @@ script:
 
 matrix:
   include:
-    - go: 1.8.x
-      env: GOAPP=true
     - go: 1.9.x
       env: GOAPP=true
     - go: 1.10.x

vendor/google.golang.org/appengine/internal/api.go

@@ -58,8 +58,11 @@ var (
 
 	apiHTTPClient = &http.Client{
 		Transport: &http.Transport{
-			Proxy: http.ProxyFromEnvironment,
-			Dial:  limitDial,
+			Proxy:               http.ProxyFromEnvironment,
+			Dial:                limitDial,
+			MaxIdleConns:        1000,
+			MaxIdleConnsPerHost: 10000,
+			IdleConnTimeout:     90 * time.Second,
 		},
 	}