build(travis): Drop Node 0.12 build, unclear if supported by babel 6
build(travis): Drop Node 0.12 build, unclear if supported by babel 6

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