Yes, FSharp.Data.DesignTime is .NET 4.5, but that doesn't mean your projects have to be .NET 4.5 also, it's just a design time component, and you shouldn't even reference it, only FSharp.Data.dll
EDIT:
In FSharp.Data 1.1.x the DesignTime dll was being added by mistake when referencing the nuget package, in 2.x that is now fixed