Add ssl_ciphers for compatability

[pezzak]

On some browsers(e.g: Chromium Version 49.0.2623.108 Ubuntu 14.04 (64-bit), Firefox version 47) redmine does not open. In chromium i receive 'spdy inadequate transport security' error.
After changing ssl_ciphers in nginx to 'ECDHE-RSA-AES128-GCM-SHA256:AES256+EECDH:AES256+EDH' everything works.
I have found solution here: http://www.jeffgeerling.com/blog/2016/fixing-errspdyinadequatetransportsecurity-ssl-error-chrome