Add scala to the filetype

The companion to derekwyatt/vim-scala@e6e1237. This triggers other parts
of vim-scala's ftplugin to load on sbt files that are useful, like
default shiftwidth/softtabstop, etc. It will also mean that the
filetype is set to `sbt.scala` no matter whether vim-sbt or vim-scala
loads first.

ftdetect/sbt.vim

@@ -3,4 +3,4 @@
 " Maintainer:   Derek Wyatt <derek@{myfirstname}{mylastname}.org>
 " Last Change:  2012 Jan 19
 
-au BufRead,BufNewFile *.sbt set filetype=sbt
+au BufRead,BufNewFile *.sbt set filetype=sbt.scala