build: handle prerelease versions in deletion target rule

[crisbeto]

Currently the deletion target rule doesn't handle comparing something like 6.0.0-beta.2 and 6.0.0. With these changes both versions will be considered the same.

Note: lint failures are to be expected, because the rule now compares the beta version correctly. We may have to delay merging this in until we've done all the breaking changes.

[jelbourn]

Yeah, we should hold off on merging this until we remove the deprecated stuff.

[crisbeto]

@jelbourn this one is unblocked now.

[jelbourn]

LGTM

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "continuous-integration/travis-ci/pr" is failing
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