tls: update default ciphers to use gcm and aes128

AES-GCM or CHACHA20_POLY1305 ciphers must be used in current version of
Chrome to avoid an 'obsolete cryptography' warning.

Prefer 128 bit AES over 192 and 256 bit AES considering attacks that
specifically affect the larger key sizes but do not affect AES 128.

PR-URL: https://github.com/iojs/io.js/pull/1660


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>
Reviewed-By: Roman Reiss <me@silverwind.io>