SBT formula fixes

The original formula downloads the bare launcher jar, then creates
a very simple 'sbt' launcher script. The 'sbt' script provided in
the tarball distribution of sbt has a bunch of useful command line
options that I want to use instead. This patch installs sbt with
the original script distributed in the tarball.

Please note that you will have to move your SBT_OPTS from
~/.sbtconfig to ~/.profile (or ~/.zshrc depending on your shell)
and add an 'export' before it so it's available to the subshell.