Set default minimum TLS version in SSLContext to TLS 1.2
Set default minimum TLS version in SSLContext to TLS 1.2

Reviewed By: yfeldblum

Differential Revision: D25527140

fbshipit-source-id: 2a2721ca4418e1a4c57f621f41716e5c8f27869c

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly