Auto build of docs for each branch

[lbdreyer]

The documentation used to be automatically built each time each branch of iris was updated.

This has now broken (sometime since last summer).

See #3341 (comment)

[pp-mo]

I checked the old versions of README.md, and the correct link was in the v2.0.0 version

I.E. https://scitools-docs.github.io/iris/master/index.html
I'm pretty sure this used to work, but no longer does.

Meanwhile, the appropriate section of travis.yml is apparently still building and pushing docs.

Checking the correct travis run on the v2.0.0 commit. it seems that doctr is running, but it has
this at line 1843: "Don't have permission to push. Not trying."

I guess this is probably the current problem ??