fix(engine): use ${trueConstant} instead of 1 in selectActivityStatisticsByQueryCriteriaSql

Column IS_ACTIVE_ of boolean type is compared to the fixed value 1 instead of database dependent property ${trueContant}.
Changing this makes the postgres specific version of this statement superfluous.
See also https://groups.google.com/forum/#!topic/camunda-bpm-dev/n5UfAajEbhc.