Write to file as binary

This prevents certain kind of encoding issues from being thrown by Ruby.
This probably started with the change in
https://github.com/colszowka/simplecov/commit/6abb79e.

Also changed a few things so one could get to running tests quicker on
various Ruby versions.
Merge pull request #3 from zbelzer/fix_output_encoding

Fix Encoding::UndefinedConversionError