Use "cross-spawn" instead of "cross-spawn-async" (#125)

Fixes the warning upon `npm install`.
"cross-spawn-async" is now deprecated: https://git.io/v6hdS

bin/now

@@ -1,7 +1,7 @@
 #!/usr/bin/env node
 import minimist from 'minimist';
 import { resolve } from 'path';
-import { spawn } from 'cross-spawn-async';
+import { spawn } from 'cross-spawn';
 import checkUpdate from '../lib/check-update';
 
 const argv = minimist(process.argv.slice(2));

package.json

@@ -61,7 +61,7 @@
     "bytes": "2.4.0",
     "chalk": "1.1.3",
     "copy-paste": "1.3.0",
-    "cross-spawn-async": "2.2.4",
+    "cross-spawn": "4.0.0",
     "docker-file-parser": "0.1.0",
     "email-prompt": "0.1.8",
     "email-validator": "1.0.5",