Incorrect IP

The IP address for SMTP/API listed on line 228 was incorrect. Changed it
from 198.61.253.48 to 50.56.21.178.
Merge pull request #100 from mailgun/chris/docs

The IP address for SMTP/API listed on line 228 was incorrect. Changed it
from 198.61.253.48 to 50.56.21.178.