Plugin Manager can not download _site plugins from github
Sounds like github changes a bit download url for master zip file.

From `https://github.com/username/reponame/zipball/master` to `https://github.com/username/reponame/archive/master.zip`.

We need to update plugin manager to reflect that change.

In the meantime, we invite users having this issue to use:

```sh
bin/plugin -install reponame -url https://github.com/username/reponame/archive/master.zip
```

For example:

```sh
bin/plugin -install paramedic -url https://github.com/karmi/elasticsearch-paramedic/archive/master.zip
```

Closes #3551
(cherry picked from commit 352d2aa)