Switch to depending on the new conda-forge libudev package for cos7

[ryanvolz]

We modify the build matrix so that two versions are build for linux64: one using cos6 (and the libudev CDT) and one using cos7 (and the new libudev conda package). The cos7 build will be preferred when possible by using a higher build number. This allows us to remove not only the dependency on the systemd CDT package but also do away with the pre- and post- scripts (because the cos6 version will only be used when the system is old enough to supply libudev.so.0 instead of libudev.so.1 anyway).

Checklist

• [x] Used a personal fork of the feedstock to propose changes
• [x] Bumped the build number (if the version is unchanged)
• [x] Reset the build number to 0 (if the version changed)
• [x] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [x] Ensured the license file is being packaged.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.