Refactor out upgrade package

upgrade/upgrade_unsupp.go

@@ -0,0 +1,15 @@
+// Copyright (C) 2014 Jakob Borg and Contributors (see the CONTRIBUTORS file).
+// All rights reserved. Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+// +build windows solaris noupgrade
+
+package upgrade
+
+func Upgrade(current string) (Release, error) {
+	return Release{}, ErrUpgradeUnsupported
+}
+
+func LatestRelease(prerelease bool) (Release, error) {
+	return Release{}, ErrUpgradeUnsupported
+}