Update ProjectFileParser.groovy

On 64 bit systems, the default variable MSBuildExtensionsPath = MSBuildExtensionsPath64.
However, the default installation path of WinPhone sdk contains MSBuildExtensionsPath32.
So plugin search imports from autogenerated *.csproj file, which contain $(MSBuildExtensionsPath), in MSBuildExtensionsPath64, but it is contained in MSBuildExtensionsPath32. With this change plugin trying to find the imports in MSBuildExtensionsPath64. Then if the exception plugin trying to find the imports in MSBuildExtensionsPath32.