Use LATEST for dev versions

This is much more robust, because the dev couplings will now _always_
point to the latest snapshot deployed by Jenkins (i.e., what's currently
on the official master branch), rather than being out of date.

More critically, using a not-latest version of pom-imagej will now no
longer cause problems with the dev couplings, because "LATEST" always
automatically refers to the latest deployed version, even if that
version differs from the one available when pom-imagej was released.

See also:
https://github.com/scijava/pom-scijava/commit/aec7fbddfbe59b3c4c32d5288de2fa8d6c9ef0f6