I'm having some trouble using the optional coxeter3 package - can somebody see what I'm doing wrong?

I just built a clean version of sage 8.0, freshly cloned from github, on my ubuntu machine. Then I ran

```
sage -i coxeter3
```

as one does; it installed with no errors. It seems to have worked: installed_packages(), for instance, has the line

```
'coxeter3': '1.1',
```

and there's a sage.libs.coxeter3 module in the namespace. But, for instance, when I try to use coxeter3 as per the instructions on the Coxeter Groups page in the sage documentation (can't provide a link as karma is too low), I get a RuntimeError:

```
sage: W = CoxeterGroup(["A",2], implementation='coxeter3')
---------------------------------------------------------------------------
RuntimeError Traceback (most recent call last)
<ipython-input-12-4f0efd988562> in <module>()
----> 1 W = CoxeterGroup(["A",Integer(2)], implementation='coxeter3')
/home/bjy/build/sage/local/lib/python2.7/site-packages/sage/combinat/root_system/coxeter_group.pyc in CoxeterGroup(data, implementation, base_ring, index_set)
142 from sage.libs.coxeter3.coxeter_group import CoxeterGroup
143 except ImportError:
--> 144 raise RuntimeError("coxeter3 must be installed")
145 else:
146 return CoxeterGroup(cartan_type)
RuntimeError: coxeter3 must be installed
```

Any help is appreciated! I don't know enough even to start troubleshooting, though I tried the usual kinds of google searches and such.