Build against ØMQ version 2.1.x.

The complete list of ØMQ versions is:

* 2.1.0
* 2.1.1
* 2.1.2
* 2.1.3
* 2.1.4
* 2.1.5
* 2.1.6
* 2.1.7
* 2.1.8
* 2.1.9
* 2.1.10
* 2.1.11