remove use of FloatMath class, use Math instead

FloatMath was deprecated in api 22, and removed in api 23
diff --git a/library/src/com/davemorrissey/labs/subscaleview/SubsamplingScaleImageView.java b/library/src/com/davemorrissey/labs/subscaleview/SubsamplingScaleImageView.java
index 3d96dd1..7328e3c 100644
--- a/library/src/com/davemorrissey/labs/subscaleview/SubsamplingScaleImageView.java
+++ b/library/src/com/davemorrissey/labs/subscaleview/SubsamplingScaleImageView.java
@@ -38,7 +38,6 @@
 import android.support.annotation.NonNull;
 import android.util.AttributeSet;
 import android.util.DisplayMetrics;
-import android.util.FloatMath;
 import android.util.Log;
 import android.util.TypedValue;
 import android.view.GestureDetector;
@@ -1747,7 +1746,7 @@
     private float distance(float x0, float x1, float y0, float y1) {
         float x = x0 - x1;
         float y = y0 - y1;
-        return FloatMath.sqrt(x * x + y * y);
+        return (float) Math.sqrt(x * x + y * y);
     }
 
     /**