Add matrix to build in different windows platforms.

Signed-off-by: David Calavera <david.calavera@gmail.com>

appveyor.yml

@@ -5,12 +5,18 @@ clone_folder: c:\gopath\src\github.com\docker\docker-credential-helpers
 
 # Build host
 
-os: Windows Server 2012 R2
-
 environment:
-  GOPATH: c:\gopath
-  CGO_ENABLED: 1
-  GOVERSION: 1.6
+    global:
+      GOPATH: c:\gopath
+      CGO_ENABLED: 1
+      GOVERSION: 1.6
+    matrix:
+      - platform: x86
+        GOARCH: 386
+        MSYS2_BITS: 32
+      - platform: x64
+        GOARCH: amd64
+        MSYS2_BITS: 64
 
 init:
   - git config --global core.autocrlf input
@@ -20,20 +26,9 @@ init:
 install:
   # Install Go 1.6.
   - rmdir c:\go /s /q
-  - appveyor DownloadFile https://storage.googleapis.com/golang/go%GOVERSION%.windows-amd64.msi
-  - msiexec /i go%GOVERSION%.windows-amd64.msi /q
-  # Install MinGW.
-  - ps: |
-      $url = "https://bintray.com/artifact/download/drewwells/generic/x86_64-5.1.0-release-win32-seh-rt_v4-rev0.7z"
-      $strFileName="C:\mingw64\bin\mingw32-make.exe"
-      If (Test-Path $strFileName){
-        Write-Host "Using cached mingw64"
-      }Else{
-        Write-Host "Fetching mingw64"
-        Invoke-WebRequest -UserAgent wget -Uri $url -OutFile ming32-64.7z
-        &7z x -oC:\ ming32-64.7z > $null
-      }
-  - set Path=c:\mingw64\bin;c:\go\bin;%Path%
+  - appveyor DownloadFile https://storage.googleapis.com/golang/go%GOVERSION%.windows-%GOARCH%.msi
+  - msiexec /i go%GOVERSION%.windows-%GOARCH%.msi /q
+  - set Path=c:\msys64\mingw%MSYS2_BITS%\bin;c:\go\bin;%Path%
   - go version
   - go env