Use $tab if >= 7.4.2237

- $tab was introduced in 7.4.2237 as an alternative to 999tab. At some point,
  999tab stop working.

plugin/bufexplorer.vim

@@ -826,7 +826,12 @@ function! s:SelectBuffer(...)
             " Was the tab found?
             if tabNbr == 0
                 " _bufNbr is not opened in any tabs. Open a new tab with the selected buffer in it.
-                execute "999tab split +buffer" . _bufNbr
+                if v:version > 704 || ( v:version == 704 && has('patch2237') )
+                  " new syntax for last tab as of 7.4.2237
+                  execute "$tab split +buffer" . _bufNbr
+                else
+                  execute "999tab split +buffer" . _bufNbr
+                endif
                 " Workaround for the issue mentioned in UpdateTabBufData
                 call s:UpdateTabBufData(_bufNbr)
             else