geckodriver: Read Firefox version by launching binary if mozversion f…

…ails (#685)

In some circumstances mozversion's attempt to read the Firefox version
from ini files can fail. This can be, for example, when the "binary"
is actaully a shell script that launches the real Firefox. In that
case it isn't necessarily possible to locate the ini files without
making the user explicitly provide a path. Instead fall back on
running `firefox -version` in this case and extracting the binary from
the returned string.

Fixes: SeleniumHQ/selenium#3884
Source-Repo: https://github.com/mozilla/geckodriver
Source-Revision: 6d1c7f28da90910c05426bbda8a70c42676033fa

committer: Andreas Tolfsen <atomozilla.com>

UltraBlame original commit: cfb7c7e11f6acd6a5b72d102317a1533f4ccd83c