Update symfony dependency, fix #1094 : XMLHTTP responses are cached under IE7
Use system composer if present
Update locales
Fix #1090 : feed entry mail notification
Fix #1103 : Read receipt are not requested anymore for push and validations
Merge pull request #317 from romainneutron/3.7.7

[3.7.7] latest fixes
Bump to version 3.7.7
Fix #1041 Keep video aspect ratio when extracting thumbnail using canva