Fix _mpld3_setup.py for python > 3.3

Hi Jake,
First of all thanks for mpld3!

This PR solves a problem I'm having while trying to install it under python 3.3.

PS: I'm having some issues to [package](https://build.opensuse.org/package/show/home:ocefpaf/python3-mpld3) mpld3 for openSUSE due to the complex submodule scheme you chose.

It is nice, at least for packagers, that the submodules are not updated (nor checked/touched) at setup.py.  It would be preferable that they became a separated module, making it a dependency.

If that is not possible, an alternative is creating an if/clause inside setup.py that do not touch the submodules if one is not installing from a repository.  Making updating the submodules a packagers responsibility and not something that happens automagically.