fix(travis): use the private assets only when building branches from main repository
fix(travis): use the private assets only when building branches from main repository

simonft/nylas-mail,nylas-mail-lives/nylas-mail,simonft/nylas-mail,nylas-mail-lives/nylas-mail,nirmit/nylas-mail,nirmit/nylas-mail,simonft/nylas-mail,nirmit/nylas-mail,nirmit/nylas-mail,nylas-mail-lives/nylas-mail,simonft/nylas-mail,nylas-mail-lives/nylas-mail,nylas-mail-lives/nylas-mail,nylas/nylas-mail,nylas/nylas-mail,nirmit/nylas-mail,simonft/nylas-mail,nylas/nylas-mail,nylas/nylas-mail,nylas/nylas-mail