add a 301 status code as acceptable code

Dear sindresorhus,

I have a little fixed a grunt-php code.

My project have a index.php at the document root, and it make a 301 redirection to move to the sub folder /ja, /en and so on. 
Then grunt-php create a php server repeatedly.

This is my first pull request, by the way. Please check the code and merge if it is OK.

Motoyasu Meguro. 
`if (acceptStatus.indexOf(res.statusCode) !== -1) {`