add stop to clean up gocache

.travis.yml

@@ -1,19 +1,20 @@
 sudo: false
 language: go
 go:
-- 1.10.x
-- 1.11.x
-- 1.12.x
-- tip
+  - 1.10.x
+  - 1.11.x
+  - 1.12.x
+  - tip
 
 before_install:
-- export PATH=/home/travis/gopath/bin:$PATH
+  - export PATH=/home/travis/gopath/bin:$PATH
 
 install:
-- go get -u ./weed/...
+  - rm -rf `go env GOCACHE`
+  - go get -u ./weed/...
 
 script:
-- go test ./weed/...
+  - go test ./weed/...
 
 before_deploy:
   - make release