update to support Mailgun 1.7.1+

mailgun/mailgun-php 1.7.1 now uses guzzle >=3.8, but if you already have guzzle 3.9 installed it errors out as mailgun/mailgun-php 1.7 only uses guzzle 3.8
Merge pull request #18 from codivist/patch-1

Update to support Mailgun 1.7.*. Since mailgun/mailgun-php 1.7.1 there is a less restrictive Guzzle requirement.