Can't build due to TLS v1.2 support missing in Boost 1.53

[iamthebot]

9701a45 breaks compatibility with Boost 1.53 (the latest boost version on RHEL/CentOS 7.3). Luckily, the underlying openssl library does support it so we can still mandate TLS 1.2 support via OpenSSL flags.

This is now fixed by PR #116

[iamthebot]

#116 has now been refactored to use a compile-time fallback.