Remove custom media type for Repository Invitations API

[gmlewis]

GitHub Announcement:
https://developer.github.com/changes/2017-08-23-repository-invitation-api/

The custom media type mediaTypeRepositoryInvitationsPreview that was added in #373 can now be removed as it is no longer necessary.

Edit: ~~~GitHub Enterprise API customers may wish to comment on this issue if they feel that this removal should be delayed for any reason.~~~

[RaviTezu]

Hi @gmlewis Can I take this?

[gmlewis]

I just talked to @willnorris who reminded me that we will not be removing any features until we get confirmation that the removal won't break Enterprise API clients... so please hold off on addressing this issue.

Also, semantic versioning, once we nail that down for this repo, should assist with this situation.

See #376, #617, and #638 for further discussion.

[RaviTezu]

Sure @gmlewis

[kshitij10496]

I can take this up next if @RaviTezu is not currently working on it.

[gmlewis]

Please see my comment above. We need confirmation from GitHub Enterprise API customers that the removal will not break them, which has happened in the past. Alternatively, semantic versioning needs to be adhered to in this repo before proceeding with this issue.

[knrt10]

@gmlewis is this issue still blocked? If not I would like to work on this.

[gmlewis]

This issue is no longer blocked thanks to versioning. Feel free to take it, @knrt10.

[knrt10]

Would you please assign me.

[gmlewis]

You first need to be sent an invite, @knrt10, and @willnorris can send you one when he has a chance.

In the meantime, however, please consider this issue to be assigned to you.
Others will see that in this comment section.