Support a less vague pushing policy

I don't know entirely why, but in my `pod push` it dies when trying to push during the deploy. It could be this

```
[push]
	default = simple
``` 

in my config, it could be that I have an updated version of git. It could be a lot of things, as it seems to be related to tracking branches as I only have one remote on my master repo.

```
~/./r/master (master=) git remote
origin
~/./r/master (master=) pwd
/Users/orta/.cocoapods/repos/master
```

This fixes it. Not entirely sure the implications. So opened a PR.


Fixes https://github.com/CocoaPods/CocoaPods/issues/1446

Fixes https://github.com/CocoaPods/CocoaPods/issues/1529