Provide Option for PHP version with composer:install_executable

Not all systems have a binary on the path for `php`. Systems with multiple PHP versions installed generally append the version number to the end. For example:

```
which php72
/usr/bin/php72

which php56
/usr/bin/php56
```

To work around this an option that allows the PHP version to be specified works. This could be along the lines of:

```
set :composer_php_version, :php72
````

or

```
set :composer_php_version, '/usr/bin/php72'
```

The default version would always be `:php`

Related issues:
* https://github.com/capistrano/composer/issues/46 - This one tries to use php56 (but with args).