Stop the deprecation warning.

I swear this was needed for some platofmr so I guess we'll find out.
If so, we must document why.

Author: greggman

build/execute.js

@@ -5,7 +5,7 @@ const debug = DEBUG('execute');
 export function execute(cmd, args, options) {
   return new Promise((resolve, reject) => {
     debug(`${cmd} ${args.join(' ')}`);
-    const proc = spawn(cmd, args, {...options || {}, shell: true, stdio: 'inherit'});
+    const proc = spawn(cmd, args, {...options || {}, stdio: 'inherit'});
     proc.on('close', function(code) {
       const result = {exitCode: code};
       if (parseInt(code) !== 0) {