improved Composer support

This PR just fixes to minor issues:

* We should not advocate using ```dev-master``` to users, or they create apps that **will break** at some point. Instead, using a stable version is more suitable (especially since there is a very recent one available). To not change the ```README.md``` all the time, the text just directs users to Packagist to pick whatever is stable at that moment.
* If someone really needs untested stuff, ```dev-master``` is still a bad idea (think of Pheanstalk 3.x with an incompatible API). This PR adds an alias for the master branch, so people can require the virtual version ```2.x-dev``` (or ```2.*@dev```). When changing the API in master, one can simply open up an actual  ```2.x``` branch and everything continues to work.