dlang dub.json dependency for openssl is too restrictive

In reviewing the dub.json file it specifies this deimos openssl dependency:

    "openssl": {
      "version": "~>1.1.6"
    }

This limits package dependencies to native OpenSSL 1.0 systems only. The correct dependency should be:

    "openssl": {
      "version": ">=1.1.6"
    }

This allows the consumer to use either the v1.1.6 (OpenSSL 1.0 compatible) or v2.0.0 (OpenSSL 1.1 compatible) version of the OpenSSL deimos package.

The `systemDependencies` field should be updated to reflect this as well.