fixed session credentials for the traffic example

this should be *either*:

d = Dyn::Traffic::Client.new('yourcustomername', 'youruser', 'yourpassword')

*or*

customername = 'customername'
username = 'yourusername'
password = 'yourpassword'
Dyn::Traffic::Client.new(customername, username, password)

to avoid unknown variable warnings. Thank you!