Go back to using mainine velocity

We moved off to our own fork of velocity many moons ago to fix
a memory leak bug when velocity was being barely maintained. They
have now merged the bugfix, so go back to mainline.

src/VelocityBounce.js

@@ -1,4 +1,4 @@
-const Velocity = require('velocity-vector');
+const Velocity = require('velocity-animate');
 
 // courtesy of https://github.com/julianshapiro/velocity/issues/283
 // We only use easeOutBounce (easeInBounce is just sort of nonsensical)