Rename start & end properties to fix conflict with RN 51

A recent commit in RN added start/end properties to RCTViewManager.
Because LinearGradient inherits from this class and has properties of
the same name, RN gets mixed up and tries to convert to a YGValue
instead of CGPoint, which results in the exception:

```
Error setting property 'start' of BVLinearGradient with tag #18: JSON
value '{
    x = 0;
    y = 0;
}' of type NSMutableDictionary cannot be converted to a YGValue.
```

BVLinearGradient/BVLinearGradient.m

@@ -24,14 +24,14 @@
   self.gradientLayer.colors = colors;
 }
 
-- (void)setStart:(CGPoint)start
+- (void)setStartPoint:(CGPoint)startPoint
 {
-  self.gradientLayer.startPoint = start;
+  self.gradientLayer.startPoint = startPoint;
 }
 
-- (void)setEnd:(CGPoint)end
+- (void)setEndPoint:(CGPoint)endPoint
 {
-  self.gradientLayer.endPoint = end;
+  self.gradientLayer.endPoint = endPoint;
 }
 
 - (void)setLocations:(NSArray *)locations

BVLinearGradient/BVLinearGradientManager.m

@@ -19,8 +19,8 @@ RCT_EXPORT_MODULE();
 }
 
 RCT_EXPORT_VIEW_PROPERTY(colors, NSArray);
-RCT_EXPORT_VIEW_PROPERTY(start, CGPoint);
-RCT_EXPORT_VIEW_PROPERTY(end, CGPoint);
+RCT_EXPORT_VIEW_PROPERTY(startPoint, CGPoint);
+RCT_EXPORT_VIEW_PROPERTY(endPoint, CGPoint);
 RCT_EXPORT_VIEW_PROPERTY(locations, NSArray);
 
 @end

android/src/main/java/com/BV/LinearGradient/LinearGradientManager.java

@@ -13,8 +13,8 @@ public class LinearGradientManager extends SimpleViewManager<LinearGradientView>
     public static final String REACT_CLASS = "BVLinearGradient";
     public static final String PROP_COLORS = "colors";
     public static final String PROP_LOCATIONS = "locations";
-    public static final String PROP_START_POS = "start";
-    public static final String PROP_END_POS = "end";
+    public static final String PROP_START_POS = "startPoint";
+    public static final String PROP_END_POS = "endPoint";
     public static final String PROP_BORDER_RADII = "borderRadii";
 
     @Override

index.android.js

@@ -89,8 +89,8 @@ export default class LinearGradient extends Component {
         <NativeLinearGradient
           style={{position: 'absolute', top: 0, left: 0, bottom: 0, right: 0}}
           colors={colors.map(processColor)}
-          start={convertPoint('start', start)}
-          end={convertPoint('end', end)}
+          startPoint={convertPoint('start', start)}
+          endPoint={convertPoint('end', end)}
           locations={locations ? locations.slice(0, colors.length) : null}
           borderRadii={borderRadiiPerCorner}
         />

index.ios.js

@@ -72,8 +72,8 @@ export default class LinearGradient extends Component {
       <NativeLinearGradient
         ref={(component) => { this.gradientRef = component; }}
         {...otherProps}
-        start={convertPoint('start', start)}
-        end={convertPoint('end', end)}
+        startPoint={convertPoint('start', start)}
+        endPoint={convertPoint('end', end)}
         colors={colors.map(processColor)}
         locations={locations ? locations.slice(0, colors.length) : null}
       />