Added rsocket-cpp dependency to Ubuntu 16.04 build script

[orishu]

The Travis build started failing since fbthrift became dependent of rsocket-cpp. Sample failure: https://travis-ci.org/facebook/mcrouter/builds/489841577 Added a new recipe for rsocket-cpp and included it in the Ubuntu 16.04 scripts. Test Plan: ./mcrouter/scripts/install_ubuntu_16.04.sh "$(pwd)"/mcrouter-install succeeded on an Ubuntu 16.04 VM.

[facebook-github-bot]

Thank you for your pull request. We require contributors to sign our Contributor License Agreement, and yours has expired.

Before we can review or merge your code, we need you to email [email protected] with your details so we can update your status.