Ensure currentSlide is treated as an integer in getTarget().

Hello.

Discovered an issue in the `getTarget()` method, where, in certain instances, the `currentSlide` property is treated as a string instead of an integer, sometimes resulting in concatenation instead of addition.

Ex:
```
slider.currentSlide = 8
slider.getTarget('next')
// returns 81, instead of 9
```

My use case was very similar to that reported in #875 (reported 2 months ago, bummer). Jumping to a slide using the `flexAnimate()` method then broke on `getTarget('next')` because a concatenated string was passed instead of an integer.