Fix wrong order of lerp when parsing of color stops (#324)

The `_getColorInBetweenColorStops` interpolated between color stops with
the order of stops inverted between the two it should interpolate with
(comparing with `_getColorInBetweenOpacityStops` confirms this as well).
This means that if there is more than two stops in the gradient, the
second color in the gradient never appears and the third one takes its
place, and the colors are on the wrong stop after that.

lib/src/parser/gradient_color_parser.dart

@@ -172,7 +172,7 @@ class GradientColorParser {
       var upperColor = colorStopColors[i];
       var lowerColor = colorStopColors[i - 1];
       return GammaEvaluator.evaluate(
-              percentage, upperColor.withOpacity(1), lowerColor.withOpacity(1))
+              percentage, lowerColor.withOpacity(1), upperColor.withOpacity(1))
           .withOpacity(opacity);
     }
     throw Exception('Unreachable code.');