Do not update composer [Closes #309]

- I guess in most cases it is unwanted: When I specify :composer_bin, I want to use _this_ copy, not another one
- It doesn't make much sense, because if there is a single new commit to composer, it is nearly the
  same operation as `composer_get`. So a user _must_ keep their composer copy always up to date to avoid
  the unwanted update.
- It may have unwanted side effects, especially when `composer.phar` is shared between user.
- it may not be possible, if the user is not allowed to write to `composer.phar`