Provide Option for PHP version with composer:install_executable

[rsiddle]

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:

• #46 - This one tries to use php56 (but with args).