Make default filter case insensitive

We had an issue where for one developer SimpleCov just wouldn't work.   Turns out that in his terminal he had cded to c:\... instead of C:\... (Note the lowercase vs capital C).   Well Windows happiliy changes the prompt to lowercase.  This causes SimpleCov.root to start with a lowercase c:/.  And that causes the default filter regex to fail for every file.  Making the regex case insensitive solves this problem.  May also solve several other issues such as #77.
Merge pull request #280 from ryanatball/patch-1

Make default filter case insensitive