Lastest release dies with "Your requirements could not be resolved" errors

I've been using your package for a while happily, but the latest release is not working for me at all even on composer.json files that have worked previously. I can't get drush composer update to work at all on alpha14.

Using a composer.json file I've attached to a module previously,

{
    "require": {
        "phpunit/phpunit": "3.7.*",
        "phpunit/php-invoker": ">=1.1.0,<1.2.0"
    }
}

I die with the errors

drush composer-json-rebuild; drush composer update
Completed building composer.json file.                                                                                   [ok]
Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - The requested package phpunit/phpunit 3.7.* could not be found.
  Problem 2
    - The requested package phpunit/php-invoker >=1.1.0,<1.2.0 could not be found.

Potential causes:
 - A typo in the package name
 - The package is not available in a stable-enough version according to your minimum-stability setting
   see  for more details.

Read  for further common problems.

If I go into ~/.drush/ and replace the installed copy of ~/.drush/composer with a copy from September, drush composer update works again on exactly the same data.