[vim] prevent duplicate bufferline

Without the option:

    let g:bufferline_echo = 0

bufferline will echo the buffer list to the command bar. Since
vim-airline is also in use (and it too displays the buffer list), we get
duplicated information. By setting the above option we get the buffer
list only inside the airline (status line).

vimrc

@@ -85,6 +85,8 @@ set nocompatible
     let g:airline_symbols.linenr = '␤'
     let g:airline_symbols.whitespace = '✗'
 
+    let g:bufferline_echo = 0
+
     colorscheme solarized
     filetype plugin indent on
     syntax enable