rework modules.mac_brew.latest_version to work around brew version inconsistency

[jfindlay]

What does this PR do?

Brew lists package versions differently for packages that are not installed vs installed if the version includes a package revision.

What issues does this PR fix or reference?

Previous Behavior

   -> integration.states.pkg.PkgTest.test_pkg_008_latest_with_epoch  ................................
       Traceback (most recent call last):
         File "/opt/salt/lib/python2.7/site-packages/salttesting/helpers.py", line 68, in wrap
           return caller(cls)
         File "/testing/tests/integration/states/pkg.py", line 328, in test_pkg_008_latest_with_epoch
           self.assertSaltTrueReturn(ret)
         File "/testing/tests/integration/__init__.py", line 1456, in assertSaltTrueReturn
           **(next(six.itervalues(ret)))
       AssertionError: False is not True. Salt Comment:
       The following packages failed to update: bash-completion

New Behavior

test_pkg_008_latest_with_epoch (integration.states.pkg.PkgTest)
[CPU:48.2%|MEM:49.1%|Z:0]  ... ok

Tests written?

Yes