[Animated] Remove rebound dependency

Summary:
We already reimplement the spring computation, we were only using rebound for the tension/friction conversion. Turns out that it is quite small so we can just embed it. I'm doing this as I'm preparing for doing a web version of the feature and I'm trying to minimize the number of dependencies.

https://github.com/facebook/rebound-js/blob/master/rebound.js#L932

Libraries/Animation/Animated/Animated.js

@@ -17,12 +17,12 @@ var InteractionManager = require('InteractionManager');
 var Interpolation = require('Interpolation');
 var React = require('React');
 var Set = require('Set');
+var SpringConfig = require('SpringConfig');
 var Text = require('Text');
 var View = require('View');
 var invariant = require('invariant');
 
 var flattenStyle = require('flattenStyle');
-var rebound = require('rebound');
 var requestAnimationFrame = require('requestAnimationFrame');
 
 import type InterpolationConfigType from 'Interpolation';
@@ -351,12 +351,12 @@ class SpringAnimation extends Animation {
         config.tension === undefined && config.friction === undefined,
         'You can only define bounciness/speed or tension/friction but not both',
       );
-      springConfig = rebound.SpringConfig.fromBouncinessAndSpeed(
+      springConfig = SpringConfig.fromBouncinessAndSpeed(
         withDefault(config.bounciness, 8),
         withDefault(config.speed, 12),
       );
     } else {
-      springConfig = rebound.SpringConfig.fromOrigamiTensionAndFriction(
+      springConfig = SpringConfig.fromOrigamiTensionAndFriction(
         withDefault(config.tension, 40),
         withDefault(config.friction, 7),
       );

Libraries/Animation/Animated/SpringConfig.js

@@ -0,0 +1,102 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @providesModule SpringConfig
+ * @flow
+ */
+
+'use strict';
+
+type SpringConfigType = {
+  tension: number,
+  friction: number,
+};
+
+function tensionFromOrigamiValue(oValue) {
+  return (oValue - 30) * 3.62 + 194;
+}
+
+function frictionFromOrigamiValue(oValue) {
+  return (oValue - 8) * 3 + 25;
+}
+
+function fromOrigamiTensionAndFriction(
+  tension: number,
+  friction: number,
+): SpringConfigType {
+  return {
+    tension: tensionFromOrigamiValue(tension),
+    friction: frictionFromOrigamiValue(friction)
+  };
+}
+
+function fromBouncinessAndSpeed(
+  bounciness: number,
+  speed: number,
+): SpringConfigType {
+  function normalize(value, startValue, endValue) {
+    return (value - startValue) / (endValue - startValue);
+  }
+
+  function projectNormal(n, start, end) {
+    return start + (n * (end - start));
+  }
+
+  function linearInterpolation(t, start, end) {
+    return t * end + (1 - t) * start;
+  }
+
+  function quadraticOutInterpolation(t, start, end) {
+    return linearInterpolation(2 * t - t * t, start, end);
+  }
+
+  function b3Friction1(x) {
+    return (0.0007 * Math.pow(x, 3)) -
+      (0.031 * Math.pow(x, 2)) + 0.64 * x + 1.28;
+  }
+
+  function b3Friction2(x) {
+    return (0.000044 * Math.pow(x, 3)) -
+      (0.006 * Math.pow(x, 2)) + 0.36 * x + 2;
+  }
+
+  function b3Friction3(x) {
+    return (0.00000045 * Math.pow(x, 3)) -
+      (0.000332 * Math.pow(x, 2)) + 0.1078 * x + 5.84;
+  }
+
+  function b3Nobounce(tension) {
+    if (tension <= 18) {
+      return b3Friction1(tension);
+    } else if (tension > 18 && tension <= 44) {
+      return b3Friction2(tension);
+    } else {
+      return b3Friction3(tension);
+    }
+  }
+
+  var b = normalize(bounciness / 1.7, 0, 20);
+  b = projectNormal(b, 0, 0.8);
+  var s = normalize(speed / 1.7, 0, 20);
+  var bouncyTension = projectNormal(s, 0.5, 200);
+  var bouncyFriction = quadraticOutInterpolation(
+    b,
+    b3Nobounce(bouncyTension),
+    0.01
+  );
+
+  return {
+    tension: tensionFromOrigamiValue(bouncyTension),
+    friction: frictionFromOrigamiValue(bouncyFriction)
+  };
+}
+
+module.exports = {
+  fromOrigamiTensionAndFriction,
+  fromBouncinessAndSpeed,
+};