Author: Sam Hemelryk
MDL-45412 cachestore_mongodb: added notification of old Mongo version use
http://git.moodle.org/gw?p=moodle.git;a=commit;h=6deb0813d0cac785e296930bf1993c9d0105d305
http://github.com/moodle/moodle/commit/6deb0813d0cac785e296930bf1993c9d0105d305
+ 2.7 en [pleaseupgrademongo,cachestore_mongodb]