Hamdi/challenge vulnerability

[hamdiallam]

Ensure that the txBytes used to challenge both deposits/transactions are direct spends of the exiting transaction. Using the grandchild transaction will not work because the confirm signature needs to sign over the spending the transaction.

No tests as this is a simple require statement.

Fixes #63

Changed fastforward helper in the test removing the strict bound. The difference in time between the mined block and block before the fastfoward only needs to be atleast time or bigger

[coveralls]

Pull Request Test Coverage Report for Build 396

• 14 of 14 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 92.283%

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Totals
Change from base Build 392:	0.05%
Covered Lines:	207
Relevant Lines:	217

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💛 - Coveralls