Require forked version of phing-drush-task

[froboy]

Drupal 8.4 requires Symphony 3 which isn't currently supported in Drush 8. See drush-ops/drush#2724 for more. phing-drush-task is locked to drush < 8.1, so I forked it and changed composer.json, but I ran into some composer issues getting the forked library to load. Instead of trying to debug PSR-0 I just rewrite the handful of phing drush commands in the build with straight exec commands which run in the directory set by the build variables.

This also removes php-todo-finder https://github.com/nilportugues/php-todo-finder/blob/master/composer.json as it's requiring old Symfony libraries and and doesn't seem to be used.

@becw Drupal 8.4 is not installable with our build tools until this or a similar fix is accepted.

[becw]

An alternative to this approach is to alias the version of Drush required by palantirnet/drupal-skeleton. Since the dependency issue is ultimately a conflict between the dependencies of that repository, I think that approach introduces fewer moving parts/things to update later. Here's how that change looks: palantirnet/drupal-skeleton#70

[becw]

I've created a new branch based on this work - see PR #85. I found that:

• Command syntax has changed for some commands in Drush 9
• When removing the nilportugues/php_todo library, I needed to remove the associated functionality
• Updating the continuousphp/phing-drush-task library for Drush 9 was pretty straightforward

[becw]

Also, it turns out this update breaks backward compatibility!

[becw]

The remote branch has been merged, so this should no longer be an issue.

I'm going to leave the branch in case anyone was using it.