Fix import errors of get_package_verion and get_version_string

On a fresh "git clone" of the repository, running postreview.py will bail out because it can't find get_package_version or get_version_string.  This patch simply adds the script's parent directory to the search path.

Tested that the following sequence works on Linux without errors:

git clone git://github.com/reviewboard/rbtools.git
export PATH="${PATH}:$(pwd)/rbtools/rbtools"
apply patch
git add rbtools/rbtools/postreview.py
git ci
postreview.py --guess-summary --guess-description