Update fabric.curvedText.js

- I've corrected the alignment calculation for proper angle calculation
- Changed the 'letters' group positioning to manual calculation (this.letters.left = this.letters.left - this.width / 2; instead of this.letters._updateObjectsCoords();) since it cause misplacement
- Disabled the custom 'render' function, since it created a ghost object

Used and tested with fabricjs v1.4.9