git_version wasn't expecting a version number that contained letters, LOL

My whacked-out installed git from DTurner's mad science lab has version "number" 1.9.1.598.g9119e8b .
git_version's regexp wasn't expecting letters mixed in there.
So this test was crashing, trying to call None.group(2)

I changed the behavior: get the part of the version that looks like
dotted numbers. Get the 1.9.1.598, drop the letter-y part.

Testing Done:
Now the test doesn't crash.

(The test fails for me running DTurner's mad science git.
DTurner knows and is saying things that make me think he
knows what's going on, yay)

Reviewed at https://rbcommons.com/s/twitter/r/456/