update html links in README to point to new arbor-sim (#628)

fixes #627

README.md

 # Arbor Library
 
-[![Build Status](https://travis-ci.org/eth-cscs/arbor.svg?branch=master)](https://travis-ci.org/eth-cscs/arbor)
+[![Build Status](https://travis-ci.org/arbor-sim/arbor.svg?branch=master)](https://travis-ci.org/arbor-sim/arbor)
 
 Arbor is a library for implementing performance portable network simulations of multi-compartment neuron models.
 
 An installation guide and library documentation are available online at [Read the Docs](http://arbor.readthedocs.io/en/latest/).
 
-[Submit a ticket](https://github.com/eth-cscs/arbor) if you have any questions or want help.
+[Submit a ticket](https://github.com/arbor-sim/arbor) if you have any questions or want help.