adjust postinstall command so that it works in Windows as well as *nix (#8744)

Semicolons in postinstall commands don't work in Windows.

'&&' works in *nix and in at least some versions of Windows.

This changes the meaning of the postinstall line slightly because
now the later commands won't run if the earlier ones failed but I
don't see that being a problem.

package.json

@@ -159,7 +159,7 @@
     "client:e2e": "node test/client/e2e/runner.js",
     "client:test": "npm run client:unit && npm run client:e2e",
     "start": "gulp run:dev",
-    "postinstall": "bower --config.interactive=false install -f; gulp build; npm run client:build"
+    "postinstall": "bower --config.interactive=false install -f && gulp build && npm run client:build"
   },
   "devDependencies": {
     "babel-plugin-istanbul": "^4.0.0",