Strip "\n" from end of OS version string for OS X.
Strip "\n" from end of OS version string for OS X.

If you don't, it ends up in the UserAgent header and you get an error
like:
Invalid header value 'CertbotACMEClient/0.8.0 (darwin 10.10.5\n)...'

letsencrypt/letsencrypt,letsencrypt/letsencrypt,jtl999/certbot,jtl999/certbot,jsha/letsencrypt,lmcro/letsencrypt,bsmr-misc-forks/letsencrypt,bsmr-misc-forks/letsencrypt,lmcro/letsencrypt,stweil/letsencrypt,jsha/letsencrypt,stweil/letsencrypt