Add astor to install_requires.

`hy --spy` fails on hy 0.9.11.

    $ hy --spy
    hy 0.9.11
    => (type "hy")
    Traceback (most recent call last):
      File "/usr/local/lib/python2.7/dist-packages/hy/cmdline.py", line 68, in print_python_code
        import astor.codegen
    ImportError: No module named astor.codegen