Git-finder checks minor OS X version number

Previously it looked for the version string "10.9" specifically, which
was really stupid of me. Now it looks specifically at the second part
part of the version string — the "9" in Mavericks or the "10" in
Yosemite — and tests it against the set of values ("9", "10"). When
10.11 comes out we'll just make it ("9", "10", "11").