Add ability to detect current installed version of GHE

[bitwiseman]

Description

It looks like as far back as GHE 2.12 the /meta endpoint supports returning the GHE version as installed_version.

• https://developer.github.com/enterprise/2.12/v3/meta/
• https://developer.github.com/enterprise/2.20/v3/meta/

This field is not preset for https://api.github.com/meta .

This is version could then be used to change the behavior of the library depending on GHE version. See #822 and #845 for for an example of how this would be extremely useful.

• https://developer.github.com/enterprise/2.19/v3/git/refs/
• https://developer.github.com/enterprise/2.18/v3/git/refs/

[bitwiseman]

We should keep keep the scenarios of #1143 in mind when implementing this. Previews should be bindable to specific version ranges.

[bitwiseman]

Almost all previews have be graduated. There is a new system for API versioning in GithHub. We will need to do something similar to this issue for the new scheme.