Improve appveyor config

Author: felixfbecker

appveyor.yml

@@ -1,13 +1,16 @@
 version: "{build}"
 shallow_clone: true
 clone_folder: c:\gopath\src\github.com\sourcegraph\cloneproxy
+skip_branch_with_pr: true
+
 environment:
   GOPATH: c:\gopath
+
 install:
   - set PATH=%GOPATH%\bin;c:\go\bin;%PATH%
-  - echo %PATH%
-  - echo %GOPATH%
   - go version
-  - go env
-build_script:
-  - go test -v ./...
+
+build: off
+
+test_script:
+  - go test -v ./...