release: allow THIS_VERSION to be defined in another file

The release task updates the field 'THIS_VERSION' in the buildfile itself.

If you have a project that maintains multiple branches of a product (say 1.x and 2.x), this leads to a non-trivial merge conflicts in an essential file.

It would be convenient if the version could be specified in a separate file. That way we can revert the file on each merge with less chance of introducing an error.