Bug 993377: pin versions of OpenSSL

This requires OpenSSL everywhere, so that its version can be controlled.
It's already installed everywhere, as a prerequisite for one package or
another.

 - OS X: just ensure it's present
 - CentOS: pin versions for supported releases
 - Ubuntu: still to do, so just ensure present

The CentOS 6.5 version is pinned to one not vulnerable to Heartbleed.