RTL css fix: only apply when dir=rtl

**Bugged behaviour:**
![screen shot 2014-08-15 at 10 17 35](https://cloud.githubusercontent.com/assets/2545042/3931237/ab08fafc-2454-11e4-963d-052e1a5bd382.png)

**Fix:**
Changed some rtl-specific css from
```.ui-grid-header-cell:first-child .ui-grid-vertical-bar```
to
```.ui-grid[dir=rtl] .ui-grid-header-cell:first-child .ui-grid-vertical-bar,```

So now the vertical-bar in the first cell is only hidden when dir="rtl".