title: "Target F# Runtime" grayed out - why?
body: <p>I'm trying to change the project target runtime from F# 3.1 to F# 4.0 in VS2015 RTM, but the target runtime dropdown is grayed out:
  <a href="https://i.stack.imgur.com/Qb1du.png" rel="noreferrer"><img src="https://i.stack.imgur.com/Qb1du.png" alt="enter image description here"></a></p>
  
  <p>Any reason for this? How can I fix it?</p>