crypto/tls: don't select TLS 1.2 cipher suites in prior versions.

AES-GCM cipher suites are only defined for TLS 1.2, although there's
nothing really version specific about them. However, development
versions of NSS (meaning Firefox and Chrome) have an issue where
they'll advertise TLS 1.2-only cipher suites in a TLS 1.1 ClientHello
but then balk when the server selects one.

This change causes Go clients not to advertise TLS 1.2 cipher suites
unless TLS 1.2 is being used, and prevents servers from selecting them
unless TLS 1.2 has been negotiated.

https://code.google.com/p/chromium/issues/detail?id=297151
https://bugzilla.mozilla.org/show_bug.cgi?id=919677

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/13573047