Use AppVeyor's Start-FileDownload cmdlet

Start-FileDownload maintains current directory context and allows
specifying a request timeout, see [1].

[1] https://www.appveyor.com/docs/how-to/download-file#start-filedownload-cmdlet

appveyor.yml

@@ -33,7 +33,7 @@ build_script:
 test_script:
 - ps: |
     $ErrorActionPreference="Stop"
-    Invoke-WebRequest https://github.com/ethomson/poxyproxy/releases/download/v0.1.0/poxyproxy-0.1.0.jar -OutFile poxyproxy.jar
+    Start-FileDownload https://github.com/ethomson/poxyproxy/releases/download/v0.1.0/poxyproxy-0.1.0.jar -FileName poxyproxy.jar
     # Run this early so we know it's ready by the time we need it
     $proxyJob = Start-Job { java -jar $Env:APPVEYOR_BUILD_FOLDER\build\poxyproxy.jar -d --port 8080 --credentials foo:bar }
     ctest -V -R libgit2_clar