Bug 863768: Fix _version.py to parse version numbers from firefox-XX tags.

Strawman:

Assuming the repo is tagged with (or archived from) firefox-XX-dev or firefox-XX-release then this generates versions like XX-dev or XX (appending -dirty if the tree has been modified). This is I think what we want for our versioning scheme except it breaks with our current firefox-XX-<date> tagging when uplifting. We could switch that to firefox-XX-dev-<date> to make this work or do some more complex changes to _version.py (it's currently a very generic script).