devel/php-composer: Update from 1.4.2 to 1.5.2

-Changelog:

https://github.com/composer/composer/releases/tag/1.4.3
https://github.com/composer/composer/releases/tag/1.5.0
https://github.com/composer/composer/releases/tag/1.5.1
https://github.com/composer/composer/releases/tag/1.5.2

Add missing mbstring module to avoid fatal PHP error

PR: 223063
Reported by: eqvvs <freebsd@flornet.de>
Approved by: Naram Qashat <cyverbotx@cyberbotx.com> (maintainer), tz (mentor)
MFH: 2017Q4
Differential Revision: https://reviews.freebsd.org/D12711