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:
:warning: this breaks a test in `project_spec:71`, you can fix it though by stubbing out the `dedupe` method to return whatever it was given

`fixtures_project.stub(:dedupe) { |coverage_files| coverage_files }`