Updated usage information for runtests.pl

As this is how it introduces itself when run without parameters and also how it works properly.

```
$ perl runtests.pl
runtests.pl SPEC PROGRAM
Set ANSI_COLORS_DISABLED=1 if you redirect to a file.
Set PATT='...' to restrict tests to sections matching a regex.
```
Merge pull request #54 from cebe/patch-1

Updated usage information for runtests.pl