fix(common.sh): Do not add dev build id when outside a repo checkout.

When running from a au zip or other strange situation assume the version
in version.txt should be used as-is. This avoid the need to set
COREOS_OFFICIAL=1 in these situations which can be lead to surprises.
Merge pull request #87 from marineam/official

fix(common.sh): Do not add dev build id when outside a repo checkout.