[BUGFIX] Add mbstring extension to composer.json
[BUGFIX] Add mbstring extension to composer.json

This helps it work on heroku, where mbstring isn't available by default.

MyIntervals/emogrifier,iFixit/emogrifier,OzzyCzech/emogrifier,Seldaek/emogrifier,ernsto/emogrifier,Synchro/emogrifier,sanderkruger/emogrifier,ricardofiorani/emogrifier,jjriv/emogrifier