update to newer libbase, and allow setting via env

Other packages have this, and it makes managing the ebuild easier, so
let the env set the libbase version to use.

BUG=chromium-os:25872
TEST=`emerge-x86-alex metrics` works
TEST=`cros_run_unit_tests --board=x86-alex -p metrics` pass
TEST=build_images+build_packages for x86-alex boots up

Change-Id: I2f417a13f8220d653aa2bcf6c89f519fb4f2e3df
Reviewed-on: https://gerrit.chromium.org/gerrit/20198


Reviewed-by: Darin Petkov <petkov@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>