Update AppVeyor config to fetch dependencies

appveyor.yml

diff --git a/appveyor.yml b/appveyor.yml
index c2866a2..006f315 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -1,10 +1,15 @@
 version: '{build}'
+clone_folder: C:\gopath\src\github.com\spf13\afero
+environment:
+  GOPATH: C:\gopath
 build_script:
 - cmd: >-
     go version
 
     go env
 
-    go build
+    go get -v github.com/spf13/afero/...
+
+    go build github.com/spf13/afero
 test_script:
-- cmd: go test -v ./...
+- cmd: go test -v github.com/spf13/afero