crypto/tls: change default minimum version to TLS 1.0.

SSLv3 (the old minimum) is still supported and can be enabled via the
tls.Config, but this change increases the default minimum version to TLS
1.0. This is now common practice in light of the POODLE[1] attack
against SSLv3's CBC padding format.

[1] https://www.imperialviolet.org/2014/10/14/poodle.html

Fixes #9364.

Change-Id: Ibae6666ee038ceee0cb18c339c393155928c6510
Reviewed-on: https://go-review.googlesource.com/1791
Reviewed-by: Minux Ma <minux@golang.org>