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:
```cf_array.max_by(&:percentage_lines_tested)``` seems to be doing something funky. I replaced that part with ```cf_array[0]``` as a sanity check, and that works fine. AFAIK ```max_by``` is supposed to do what we want but something strange is happening. :scream_cat: