Rename Formula#php_fpm_startup_plist for compatibility with brew-services

[`brew-services`](https://github.com/Homebrew/homebrew/blob/master/Library/Contributions/cmd/brew-services.rb) is a wonderful little wrapper around `launchctl` that's included with Homebrew. It helps overcome all the manual wrangling to start (and stop and restart) boot/login services.

Currently `homebrew-php` places the contents of its plist in `Formula#php_fpm_startup_plist`, meaning that `brew-services` can't install it:

```
$ brew services start php55
Error: Formula `php55` not installed, #startup_plist not implemented or no plist file found
```

If we rename to `Formula#plist`, `brew-services` can install it:

```
$ brew services start php55
==> Successfully started `php55` (label: homebrew-php.josegonzalez.php55)
```

Alternately an alias could be created, but I don't see any real reason for the usage of that particular property name, nor any real specific reliance on it.

Further work will need to be done on `brew-services` to make `brew services list` compatible with tapped repositories, but I feel like this tiny change is a good place to start.