Fix ConvertClosedToStateInMergeRequest migration

It was MySQL only. Running on PostgreSQL was resulting in:

    ==  ConvertClosedToStateInMergeRequest: migrating =============================
    rake aborted!
    An error has occurred, this and all later migrations canceled:

    PG::Error: ERROR:  operator does not exist: boolean = integer
    LINE 1: ...erge_requests" SET state = 'merged' WHERE (closed = 1 AND me...
                                                                 ^
    HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.
    : UPDATE "merge_requests" SET state = 'merged' WHERE (closed = 1 AND merged = 1)

Replacing `0` with `false` and `1` with `true` should make it both
PostgreSQL and MySQL compatible.