fix matchaddpos feature detection

* Problems summary

   Errors on many unite calls, this was produced with `:Unite -buffer-name=files buffer file_mru bookmark file_rec/async`:

```
[file_rec/async] Directory traverse was completed.
Error detected while processing function <SNR>55_call_unite_empty..unite#start..unite#start#standard..unite#view#_init_cursor..unite#view#_set_cursor_line..unite#view#_match_line:
line    1:
E117: Unknown function: matchaddpos
E15: Invalid expression: has('patch7.4.340') ? matchaddpos(a:highlight, [a:line], 10, a:id) : matchadd(a:highlight, '^\%'.a:line.'l.*', 10, a:id)
```

* Expected

   No errors.

* Environment Information
 * OS: Mac OS X
 * Vim version: MacVim via Homebrew, 7.4.273

 0. startup vim
 1. run `:Unite -buffer-name=files buffer file_mru bookmark file_rec/async`