Thank you for the pull request!

I think output_stream is a bit too long. Let's call it file, like the Python 3 built-in print(..., file=sys.stdout) function.

The Travis CI build failure is caused by PEP-8 warnings:

./objgraph.py:172:80: E501 line too long (94 > 79 characters)
./objgraph.py:201:80: E501 line too long (84 > 79 characters)
./objgraph.py:242:80: E501 line too long (85 > 79 characters)
make: *** [lint] Error 1

and renaming output_stream to file should fix two of them. The third long line will need some wrapping.