can't build chaco 4.1.0 docs without access to an X11 display

Not exactly a bug, but it does complicate rebuilding of the docs when building chaco via an automated Linux distro package build system. It seems that the chaco 4.1.0 docs can only be built with access to an X11 display:

```
$ export DISPLAY=
$ make html
sphinx-build -b html -d build/doctrees source build/html
Making output directory...
Running Sphinx v1.1.2
loading pickled environment... not yet created
building [html]: targets for 28 source files that are out of date
updating environment: 28 added, 0 changed, 0 removed
Unable to access the X Display, is $DISPLAY set properly?
make: *** [html] Error 1
```

I'm attempting the build on a Linux box running Python 2.7.2.