Allow redis ticket namespace to be set directly

Falls back to be compatible with previous behaviour when not set.

    CASClient::Tickets::Storage::Redistore.new()
    # @namespace => 'cas_'
    CASClient::Tickets::Storage::Redistore.new(:env => 'test')
    # @namespace => 'cas_test'
    CASClient::Tickets::Storage::Redistore.new(:namespace => 'foo_')
    # @namespace => 'foo_'