Workaround onLayout triggered constantly on some devices, fixes #50

This was observed reliably on a Nexus 5X with React Native 0.31+
onLayout would be triggered constantly with slightly different width and
height for the thumb size when its position changes.
As a result it triggered a re-render constantly.
This will need to be investigated on the React Native side as it's not
an expected behavior.

src/Slider.js

@@ -268,8 +268,15 @@ var Slider = React.createClass({
         <Animated.View
           onLayout={this._measureThumb}
           style={[
-            {backgroundColor: thumbTintColor, marginTop: -(trackSize.height + thumbSize.height) / 2},
-            mainStyles.thumb, thumbStyle, {left: thumbLeft, ...valueVisibleStyle}
+            {backgroundColor: thumbTintColor},
+            mainStyles.thumb, thumbStyle,
+            {
+              transform: [
+                { translateX: thumbLeft },
+                { translateY: -(trackSize.height + thumbSize.height) / 2 }
+              ],
+              ...valueVisibleStyle
+            }
           ]}
         />
         <View