After installing the Roslyn preview we found that when switching back to the standard compiler by going to tools | extensions and updates | Roslyn Preview | Disable was being ignored.
In order for disable to work properly you need to delete the following target files created by the Roslyn Preview:
Microsoft.VisualBasic.Roslyn.Targets
Microsoft.CSharp.Roslyn.Targets
Microsoft.VisualBasic.Roslyn.VisualStudio.12.0.Targets
Microsoft.CSharp.Roslyn.VisualStudio.12.0.Targets
Unfortunately you have to do this every time you switch between the Roslyn Preview and the standard compiler.