Global setting keys don't show up in autocomplete (sbtVersion)

(using 0.13.5-RC5)

```
akka > sbt
sbtBinaryVersion    sbtClearOnFailure   sbtDependency       sbtPlugin           sbtPopOnFailure     sbtResolver         sbtStashOnFailure   
akka > sbtV
{invalid input}   
akka >
```

An interesting factoid: 78 upvotes for [how to check sbt version](http://stackoverflow.com/questions/8461013/how-to-check-sbt-version).
