updated postbin link to new bin.mailgun.net url
updated postbin link to new bin.mailgun.net url

mailgun/documentation,stevenmc/documentation,mailgun/documentation,xjzhou/documentation,mailgun/documentation,kaustavha/documentation,alex/documentation,alex/documentation,stevenmc/documentation,stevenmc/documentation,kaustavha/documentation,xjzhou/documentation,kaustavha/documentation,xjzhou/documentation