Bump version.

README.md

@@ -7,7 +7,7 @@ This branch contains the prerelease of D3 4.0. This API is unstable and may chan
 If you use NPM, `npm install d3@next`. Otherwise, download the [latest release](https://github.com/mbostock/d3/releases/latest). The released bundle supports AMD, CommonJS, and vanilla environments. Create a custom build using [Rollup](https://github.com/rollup/rollup) or your preferred bundler. You can also load directly from [d3js.org](https://d3js.org):
 
 ```html
-<script src="https://d3js.org/d3.v4.0.0-alpha.18.min.js"></script>
+<script src="https://d3js.org/d3.v4.0.0-alpha.19.min.js"></script>
 ```
 
 ## API Reference

package.json

 {
   "name": "d3",
-  "version": "4.0.0-alpha.18",
+  "version": "4.0.0-alpha.19",
   "publishConfig": {
     "tag": "next"
   },