Mark installed buffer as not modified

Installer buffer is marked as modified after each installation. Mark it
as nomodified so that no warnings are shown when the buffer is closed.

autoload/vundle/installer.vim

@@ -57,6 +57,7 @@ func! vundle#installer#install(bang, name) abort
   else
     throw 'whoops, unknown status:'.status
   endif
+  set nomodified
 endf
 
 func! vundle#installer#helptags(bundles) abort