README: Update link to build status
README: Update link to build status

Had link to build status from old tests branch, updated.
gxxjjj/QuantEcon.py,dingliumath/quant-econ,andybrnr/QuantEcon.py,QuantEcon/QuantEcon.py,jviada/QuantEcon.py,QuantEcon/QuantEcon.py,jviada/QuantEcon.py,dingliumath/quant-econ,agutieda/QuantEcon.py,andybrnr/QuantEcon.py,gxxjjj/QuantEcon.py,agutieda/QuantEcon.py,chenxulong/quanteco,chenxulong/quanteco,oyamad/QuantEcon.py,mgahsan/QuantEcon.py,mgahsan/QuantEcon.py,oyamad/QuantEcon.py