mongodb session store doesn't work

[jknack]

{
  use(new Mongodb();

  session(MongoSessionStore.class);
}

We forgot to upgrade MongoSessionStore after #228

An error occurred while starting the application:
com.google.inject.CreationException: Unable to create injector, see the following errors:

1) Could not find a suitable constructor in com.mongodb.DB. Classes must have either one (and only one) constructor annotated with @Inject or a zero-argument constructor that is not private.
  at com.mongodb.DB.class(DB.java:77)
  while locating com.mongodb.DB
    for the 1st parameter of org.jooby.mongodb.MongoSessionStore.<init>(MongoSessionStore.java:131)
  at org.jooby.Jooby.lambda$38(Jooby.java:2708)