This pull request addresses the following issues.

• Licensing is currently missing from the recipe. Assuming that majority of the installations will be on clusters, information about the licence manager server would be required
• Replace url with url_for_version in case subsequent versions are added
• Create .spack dir during installation. This directory may not be created automatically causing installation failure when spack is trying to copy the log file there.
• Symlink wolframscript. Most people running mathematica in the cluster environment would be using wolframscript binary. However, the installer does not symlink it, so full path is required to run it.
• Generate spec.yaml. This file may not be created automatically causing package registration failure.