Fix the ability to generate call graph images with Webgrind and graphviz

Webgrind's default config sets the path for graphviz to `/usr/local/bin/dot`, and during provisioning, apt-get installs graphviz to `/usr/bin/dot`.

So the proposal here is to simply symlink `/usr/local/bin/dot` to `/usr/bin/dot` -- without which you can't currently generate call graphs in Webgrind.