Use IPv6/48 truncation

[stephen-soltesz]

This change updates unit tests to use IPv6/48 address truncation for IP anonymization in traceroute-caller tests.

This change depends on:

• https://github.com/m-lab/go/pull/154

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 552

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.983%

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Totals
Change from base Build 547:	0.0%
Covered Lines:	669
Relevant Lines:	697

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💛 - Coveralls