fix: logic for handling sourceRoot does not take into account process.cwd()

[bcoe]

currently the logic for handling sourceRoot does not attempt to take into account process.cwd(), this means it can't work for relative paths in most environments.

[bcoe]

CC: @Toxicable

[coveralls]

Pull Request Test Coverage Report for Build 142

• 3 of 4 (75.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 91.503%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/v8-to-istanbul.js	3	4	75.0%

Totals
Change from base Build 140:	0.04%
Covered Lines:	472
Relevant Lines:	513

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💛 - Coveralls