bash: Fix scala paths

  1. Move Scala installs from ~/.scalas/scalas to ~/.scalas
  2. Read the actual path of ~/.scalas/Current when setting up
     $SCALA_HOME and $PATH. (I think this makes it a bit easier
     to easily tell what version I'm using. The upside of the old
     method was that I just had to change where ~/.scalas/Current
     pointed to use a different version, but I rarely [if ever]
     switch between Scala installs anyway.)