Allow release backfilling

[cgewecke]

See #30

• Removes logic that prevented back-filling releases from ReleaseValidator contract
• Updates existing back fill tests to verify that latest tree logic continues to work after a back-fill occurs

[coveralls]

Pull Request Test Coverage Report for Build 107

• 9 of 9 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.9%) to 97.175%

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Totals
Change from base Build 86:	0.9%
Covered Lines:	385
Relevant Lines:	391

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💛 - Coveralls