obfsproxy shebang should point to "python2", not "python"

It currently points at "python" which is not version specific and will break horribly on systems where the default system python is python3.

This isn't a issue when it is installed with setup.py, but was when I tried a TBB nightly a few days ago. As far as I can tell every system that has python2.x installed with have a "python2" symlink so changing the shebang won't break places where this works now, but will allow it to work on more systems without breaking in horrible unintuitive ways for the user.