make the separator optional for older vim versions. resolves #239.

autoload/airline/extensions/default.vim

@@ -28,19 +28,33 @@ function! s:build_sections(builder, context, keys)
     if key == 'warning' && !a:context.active
       continue
     endif
-
+    call s:add_section(a:builder, a:context, key)
-    " i have no idea why the warning section needs special treatment, but it's
-    " needed to prevent separators from showing up
-    if key == 'warning'
-      call a:builder.add_raw('%(')
-    endif
-    call a:builder.add_section('airline_'.key, s:get_section(a:context.winnr, key))
-    if key == 'warning'
-      call a:builder.add_raw('%)')
-    endif
   endfor
 endfunction
 
+if v:version >= 704 || (v:version >= 703 && has('patch81'))
+  function s:add_section(builder, context, key)
+    " i have no idea why the warning section needs special treatment, but it's
+    " needed to prevent separators from showing up
+    if a:key == 'warning'
+      call a:builder.add_raw('%(')
+    endif
+    call a:builder.add_section('airline_'.a:key, s:get_section(a:context.winnr, a:key))
+    if a:key == 'warning'
+      call a:builder.add_raw('%)')
+    endif
+  endfunction
+else
+  " older version don't like the use of %(%)
+  function s:add_section(builder, context, key)
+    if a:key == 'warning'
+      call a:builder.add_raw('%#airline_warning#'.s:get_section(a:context.winnr, a:key))
+    else
+      call a:builder.add_section('airline_'.a:key, s:get_section(a:context.winnr, a:key))
+    endif
+  endfunction
+endif
+
 function! airline#extensions#default#apply(builder, context)
   let winnr = a:context.winnr
   let active = a:context.active