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:
:gem: :tophat: :rabbit: 

```ruby
coverage_files.group_by(&:source_file_pathname).values.map { |cf_array| cf_array.max_by(&:percentage_lines_tested) }.flatten
````

Probably wouldn't hurt to break it out onto multiple lines