add graphviz to requirements file

[gallamine]

Tried to sparse kill after 3.0.1 upgrade and got the following error:

sparse kill -n <name> -e preprod
Traceback (most recent call last):
  File "/Users/william/anaconda/envs/ds_storm_topologies/bin/sparse", line 11, in <module>
    sys.exit(main())
  File "/Users/william/anaconda/envs/ds_storm_topologies/lib/python2.7/site-packages/streamparse/cli/sparse.py", line 51, in main
    load_subparsers(subparsers)
  File "/Users/william/anaconda/envs/ds_storm_topologies/lib/python2.7/site-packages/streamparse/cli/sparse.py", line 27, in load_subparsers
    module = importlib.import_module('streamparse.cli.{}'.format(mod_name))
  File "/Users/william/anaconda/envs/ds_storm_topologies/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "/Users/william/anaconda/envs/ds_storm_topologies/lib/python2.7/site-packages/streamparse/cli/visualize.py", line 19, in <module>
    raise ImportError('The visualize command requires the `graphviz` Python '
ImportError: The visualize command requires the `graphviz` Python library and `graphviz` system library to be installed.

Pip installing graphviz fixed this issue.

[coveralls]

Coverage remained the same at 44.821% when pulling 674904bff9feb766052a2096e53281695f8c2dae on gallamine:patch-1 into 30c6d703cffe924cb7c691609a3ce0c1f96c25ad on Parsely:master.

[dan-blanchard]

Thanks for the PR! I'd actually rather make sure that graphviz is optional for people who don't want to use the sparse visualize command, so I fixed it in 5e947448d281930cbf77f7fd10ff96b67ab2107e. Will push 3.1.1 to PyPI momentarily.

This is why I should always do an install into a fresh virtualenv before releasing.