Remove build number from source, add it to .gitignore

I was getting this annoying error when checking out branches today:

    error: Your local changes to the following files would be overwritten by checkout:
	src/ggrc/settings/build_number.py
    Please, commit your changes or stash them before you can switch branches.
    Aborting

IMHO the build number shouldn't be part of the repo. This PR removes it by adding it to `.gitignore`. Now checking out branches works without a problem.