Rerun failed tests

When running tests, we now output an 'rspec.failures' file that contains information about tests that fail.  When running Pedant again with the --rerun flag, we will now just run the tests indicated in that failures file.
Remove failures file if not running with --rerun

Without this, the presence of an rspec.failures file would ALWAYS determine which tests got run.

e.g.

1. Run suite, see failures
2. Run with --rerun mode; only failed tests run
3. Fix failing tests
4. Run with --rerun mode; all failed tests now pass
5. Run entire suite; only tests listed in rspec.failures run!

Without this fix, you would need to manually remove rspec.failures; now you just don't specify the --rerun flag.
Merge branch 'cm/rerun'