fix text selection and cursor position in lines with rtl text

assume a line contain both english text and persian (rtl) text, like this
```
Sample Text متن نمونه sample text
```
atom always look at first rect size of Text Element and calculate position, this cause cursor to  place in wrong position
in our example `Range.getClientRects()` return three rects, look at this pen

http://codepen.io/farnabaz/pen/rWxrGW

src/lines-yardstick.coffee

@@ -126,4 +126,4 @@ class LinesYardstick
   clientRectForRange: (textNode, startIndex, endIndex) ->
     @rangeForMeasurement.setStart(textNode, startIndex)
     @rangeForMeasurement.setEnd(textNode, endIndex)
-    @rangeForMeasurement.getClientRects()[0] ? @rangeForMeasurement.getBoundingClientRect()
+    @rangeForMeasurement.getBoundingClientRect()