php-composer: Update to 1.10.15

Upstream Release Notes:

1.10.15
* Fixed path repo version guessing issue

1.10.14
* Fixed version guesser to look at remote branches as well as local ones
* Fixed path repositories version guessing to handle edge cases where version is different from the VCS-guessed version
* Fixed COMPOSER env var causing issues when combined with the global command
* Fixed a few issues dealing with PHP without openssl extension (not recommended at all but sometimes needed for testing)

(tpaul)