Pysubnettree

* Update Travis config for bro to zeek renaming (Daniel Thayer)

* Update for a directory name change (Daniel Thayer)

Also a few other unrelated bro to zeek renaming changes.

* Check for twine existence in `make upload` (Jon Siwek, Corelight)

* GH-14: Fix segfault in lookup operations with Python3 (Jon Siwek, Corelight)

Due to decrementing a reference count too early.

* Update the `make upload` target to use twine (Jon Siwek, Corelight)

* Update github/download link (Jon Siwek, Corelight)

* Update submodules to use github.com/zeek (Jon Siwek, Corelight)

* Update README with correct Python version requirement. (Daniel Thayer)

* Fix some issues reported by Coverity. (Daniel Thayer)