Add "set shell=/bin/bash" to boilerplate vimrc

Due to the issue #175,

Is there a reason not to put in the boilerplate vimrc?

README.md

@@ -48,6 +48,7 @@
 
    " set the runtime path to include Vundle and initialize
    set rtp+=~/.vim/bundle/Vundle.vim
+   set shell=/bin/bash
    call vundle#begin()
    " alternatively, pass a path where Vundle should install plugins
    "call vundle#begin('~/some/path/here')