Fix invalid pattern for testing msbuild availability for wp7

I have tested with a fresh installation of Windows Phone SDK 7 and I was not able to create wp7 project. The problem was with check_reqs.js which checks msbuild availability. It calls "msbuild -version" which has following output:
Microsoft (R) Build Engine Version 4.0.30319.1
[Microsoft .NET Framework, Version 4.0.30319.225]
Copyright (C) Microsoft Corporation 2007. All rights reserved.

4.0.30319.1

Current pattern does not match because of capital 'v' in 'Version'. Fix tries to match both 'Version' and 'version'.