Fix bug with double thumb background color

[sfreeman28]

#396

Fixes a bug when using double thumb slider and updating the slider value from an external input (eg TextField).
The background color of the slider is not updating correctly, this PR fixes that issue.

Here is the patch-package patch file for those interested

diff --git a/node_modules/@miblanchard/react-native-slider/lib/index.js b/node_modules/@miblanchard/react-native-slider/lib/index.js
index 379639a..ff6cf06 100644
--- a/node_modules/@miblanchard/react-native-slider/lib/index.js
+++ b/node_modules/@miblanchard/react-native-slider/lib/index.js
@@ -442,6 +442,7 @@ export class Slider extends PureComponent {
         const minTrackWidth = _startFromZero
             ? Math.abs(_value) * sliderWidthCoefficient - thumbSize.width / 2
             : interpolatedTrackValues[0];
+        const maxTrackWidth = interpolatedTrackValues[1];
         const clearBorderRadius = {};
         if (_startFromZero && _value < 0 + step) {
             clearBorderRadius.borderBottomRightRadius = 0;
@@ -455,10 +456,10 @@ export class Slider extends PureComponent {
             position: 'absolute',
             left: interpolatedTrackValues.length === 1
                 ? new Animated.Value(startPositionOnTrack)
-                : Animated.add(minThumbValue, thumbSize.width / 2),
+                : Animated.add(minTrackWidth, thumbSize.width / 2),
             width: interpolatedTrackValues.length === 1
                 ? Animated.add(minTrackWidth, thumbSize.width / 2)
-                : Animated.add(Animated.multiply(minThumbValue, -1), maxThumbValue),
+                : Animated.add(Animated.multiply(minTrackWidth, -1), maxTrackWidth),
             backgroundColor: minimumTrackTintColor,
             ...valueVisibleStyle,
             ...clearBorderRadius,