fix(downloads): Create directory path with correct separator
fix(downloads): Create directory path with correct separator

Fixes #709

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