Ensure offline repository is added to sbt-rc.

This should fix offline mode.  However, I couldn't find where/how
this was working without this fix.  This was created specifically
for offline  mode to work.   Not sure how we limped along
without it.

Review by @jamesward and @havocp.  Note for @jamesward -

I haven't had a chance to test this on windows yet. Wanted to get this to you before I disappear in meetings for the rest of the day.  See how it works for you!