Remove duplicate coverage files, favoring the file with higher coverage.

Fixes #13

:warning: Another thorough code-review for this Ruby n00b please. I kind of feel dirty using ```each``` for ```dedupe``` (instead of something more slick like ```map``` or ```uniq```) but I don't know of a nicer way of doing this de-duplication. :octopus:
I think you want `double(Slather::CoverageFile)` :+1: