Improve scroll performance of Token List

Fixes https://github.com/Uniswap/uniswap-frontend/issues/139

src/components/CurrencyInputPanel/currency-panel.scss

@@ -180,6 +180,7 @@
   }
 
   &__token-list {
+    -webkit-overflow-scrolling: touch;
     height: 17.5rem;
     overflow-y: auto;
   }