Quotation marks around MSBuild path for MSBuild 12

I was seeing this while converting my site to use Cassette:

```
C:\Program Files (x86)\MSBuild\12.0\bin\msbuild.exe Daniel15.Web\cassette.targets /p:OutputPath=Daniel15.Web\obj\Release\Package\PackageTmp\ /t:Bundle /nr:false

'C:\Program' is not recognized as an internal or external command, operable program or batch file.
```

Since MSBuild 12 (used by Visual Studio 2013) is at `C:\Program Files (x86)\MSBuild\12.0\bin\msbuild.exe`, you need to wrap the path in quotation marks.