Update golang to 1.13.4.

Signed-off-by: Lantao Liu <lantaol@google.com>

.appveyor.yml

@@ -8,8 +8,9 @@ environment:
   GOPATH: C:\gopath
   PATH: C:\go\bin;C:\tools\mingw64\bin;$(PATH)
   CGO_ENABLED: 1
+  GO111MODULE: off
   matrix:
-    - GO_VERSION: 1.12.9
+    - GO_VERSION: 1.13.4
 
 install:
   # Install Mingw

.travis.yml

@@ -3,7 +3,7 @@ git:
 
 language: go
 go:
-  - 1.12.x
+  - 1.13.4
   - tip
 
 
@@ -13,6 +13,9 @@ matrix:
 
 sudo: required
 
+env:
+  - GO111MODULE=off
+
 services:
   - docker