Get rid of the master/incoming distinction

[bstrie]

In theory, Bors should be keeping the incoming branch green at all times. Is this distinction still necessary?

Pros to getting rid of the distinction:

• Online docs only get built from master, so this keeps docs and development in sync
• There's no longer any need to tell people to target PRs at incoming
• No more questions regarding the difference between incoming and master

Cons:

• Sometimes incoming is still red despite Bors

[brendanzab]

This would be handy for library writers too.

[bstrie]

Nominating for 0.7.

[graydon]

just a bug, removing milestone/nomination.

[bstrie]

Done, closing.