Perl changes

Here are some very minor changes to the Perl runtests.pl script.

* Add the `utf8` pragma to set the proper flags on the string literals inside the script (the x and check characters)
* Require at least Perl 5.6, which is the oldest version that has all the modules and pragmas being used
* Don't buffer STDOUT, so that the final status line on STDERR (which is not buffered by default) shows up after all the other output

I also added vim `.swp` files to the gitignore