dev: use alpha as the default proxy target

webpack.config.js

  * development with webpack-dev-server.
  */
 const devServerProxyTarget
-    = process.env.WEBPACK_DEV_SERVER_PROXY_TARGET || 'https://beta.meet.jit.si';
+    = process.env.WEBPACK_DEV_SERVER_PROXY_TARGET || 'https://alpha.jitsi.net';
 
 const analyzeBundle = process.argv.indexOf('--analyze-bundle') !== -1;