Add Build for PSCore/Linux/OSX

PSCore GA is aimed for Mid-Jan, and there's quite a few bugs with 5.1 that won't be fixed (i.e. Publish-Module), so would be good to start including those in the build tasks...
PowerShellGet as a nice example of those: https://github.com/PowerShell/PowerShellGet/blob/development/.travis.yml

Start by inspecting the repository's existing build tasks and compare them with the PowerShellGet .travis.yml example linked in the issue. Determine how builds should cover PSCore on Linux and macOS, then verify that the resulting build configuration runs for those platforms and preserves the existing build behavior.

Written by the indexing model from the issue text.