Set default minimum TLS version in SSLContext to TLS 1.2

Reviewed By: yfeldblum

Differential Revision: D25527140

fbshipit-source-id: 2a2721ca4418e1a4c57f621f41716e5c8f27869c

folly/io/async/SSLContext.h

@@ -142,7 +142,7 @@ class SSLContext {
    *
    * @param version The lowest or oldest SSL version to support.
    */
-  explicit SSLContext(SSLVersion version = TLSv1);
+  explicit SSLContext(SSLVersion version = TLSv1_2);
   /**
    * Constructor that helps ease migrations by directly wrapping a provided
    * SSL_CTX*