Conditionaly upgrade utf8 to utf8mb4 for MySQL 5.5.3

See http://dev.mysql.com/doc/refman/5.5/en/charset-unicode-utf8mb4.html

As utf8mb4 is a superset of utf8, this should be transparent and backward compatible.
For those really requiring the "utf8" meant by MySQL, they can use explicitely the utf8mb3 charset.
But IMHO by default, Doctrine should really use utf8mb4, which is what everybody expect from a charset named "utf8".