Add dev packages to enable OpenSSL support in Ruby

Builds of omnibus-chef-server currently fail with

    Could not load OpenSSL.
    You must recompile Ruby with OpenSSL support or change the sources
    in your Gemfile from 'https' to 'http'.

Apparently this can be fixed by adding libssl-dev:

    https://github.com/edx/edx-platform/issues/852