Verify phpize is present

[stevegrunwell]

When installing this package for a Laravel Homestead app, I received the following output:

$ sudo ./vendor/bin/install-runkit.sh
Installing Runkit7...
Downloading Runkit7 from https://github.com/runkit7/runkit7/releases/download/1.0.5b1/runkit-1.0.5b1.tgz
13 source files, building
running: phpize
sh: 1: phpize: not found
ERROR: `phpize' failed
Runkit7 has been installed and activated!

There are two issues here:

1. We should be verifying that phpize is present — that's the php7.x-dev package for apt.
2. There should most certainly not be a success message when the installation has failed.